The sunn Forum
Sunn Musical Equipment => DIY => Topic started by: Baddog on April 17, 2011, 06:34:40 pm
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I have the schematic for the amp so I'm set there. I recaped the amp and replaced the cap can with axial lead caps. I did the totum pole thing for the first filter stage and used 500v 20uf's for the rest . The voltages are a little high in the amp but that to be expected with the raise in wal voltage over the years. Mine has 6550's power tubes with trem and reverb...
The problem I'm having is I can hear the trem pulsing with no guitar signal running through the amp. It pulses with the volume turned all the way down.
Any ideas where to start lookingfor the problem or is their a fix for it?
Thanks
BD
I forgot to say I'm new to this discussion form and sunn amps. I have a 72 fender twin , peavey mace 1/2 stack , crate vc50 3x10 , 72 bassman head and a bunch of mongrel amps I've built from old organ and tone cabinet amplifiers .
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Baddog,
Try replacing the three capacitors in the trem circuit. Two are 1 uf and one is 2 uf and they are wired in series. These are often the problem with trem 'ticking" in Fender amps, so it's worth a try.
Good Luck!
Ryan
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I'M having the problem in my sunn amp. Where you referring to the fender or the sunn trem?
Thanks
Bd
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I'M having the problem in my sunn amp. Where you referring to the fender or the sunn trem?
Thanks
Bd
Almost all of the trem circuits work the same way, regardless of the amp type. There is an oscillator that has the signal come out of the plate and then go through 3 caps with resistors to ground between each, and then the signal gets fed back into the grid of the same tube, causing positive feedback. The cap value determines the speed of the trem, with the .02uf, .01uf, .01uf setup in series being most common. As you go up in value, the speed of the trem slows down. Often when a trem circuit is not working, or if you are hearing ticking noises, then any of those 3 caps or the bypass cap on the stage can be bad. Change those caps, and check voltages on the oscillator tube and see if they are close to spec, keeping in mind the higher wall voltages affecting any voltages in the amp. Some amps will use a cathode follower stage after the oscillator to buffer the output to whatever stage they are affecting. Read the 2nd Gerald Weber book...it has a good section on trem circuits in it.
Greg
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I believe those are electrolytic caps and I changed them already. Hmmm
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Ihave another question . What should the voltages be at for the 6an8 tube? What should it be biased at ?
Mine is like the one posted below except no mid boost. I wish the schematic showed the voltages.
http://www.schematicx.com/view-schematic/sunn-sceptre-amplifier-schematic/
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The Sceptre is using transistors and some different value caps for the oscillator from what is normal on most tube amps. The Sceptre is using a 1uf, a 2uf, and a 1uf for those series caps...are these the ones you changed? They would be on the board in the amp. I don't believe they are electrolytics but its been awhile. I think its the smaller caps on the end of the board, but a pic would help....
The 6AN8 is a triode-pentode, and in this amp it is used as a cathodyne inverter, with the pentode half being the gain stage or driver, and the triode half being the splitter or inverter. I believe the voltages are a little lower than say a 200S, but not by much. The pentode half should have around 90-100v on the plate and the triode should be a bit higher...but again, its been awhile since I saw one of these and didn't write down the correct voltages. I'd look for a plate voltage around there or higher, and a cathode voltage around a volt or two...if the voltages are in those ranges, then I'd suspect a problem somewhere else, especially if you are able to get good signals on a scope out of the phase inverter....
Greg
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Thanks for the reply. I'll check the voltages his morning and post my results. Check out the schematic I posted. If your referring to the three caps that are connected to the pot via resistors . Then they are electrolytic . I did replace them with new ones. Hmmmm
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http://i702.photobucket.com/albums/ww26/baddog01/sunn%20sceptre/CIMG0274.jpg
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HI Greg
6an8 voltage readings
pin 1 300v
pin 6 100v
pin 9 .74v
pin 3 115v
New KT88's biased at 43ma at 537v
The trem gets worse with the NFB loop disconnected..... You can hear the pulse with the volume and trem controls turned all the way down . HMMMMMMM
Did you check out the pics and look at the Electrolytic on the board? They looked stock to me but I could be wrong. The sceptre schematic I have shows them being electrolytic.... What do you think I should check? Maybe one of the new I purchased where bad from the factory.... Maybe bad transistor....
Thoughts and suggestions welcome
Thanks
BD
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HI Greg
6an8 voltage readings
pin 1 300v
pin 6 100v
pin 9 .74v
pin 3 115v
New KT88's biased at 43ma at 537v
The trem gets worse with the NFB loop disconnected..... You can hear the pulse with the volume and trem controls turned all the way down . HMMMMMMM
Did you check out the pics and look at the Electrolytic on the board? They looked stock to me but I could be wrong. The sceptre schematic I have shows them being electrolytic.... What do you think I should check? Maybe one of the new I purchased where bad from the factory.... Maybe bad transistor....
Thoughts and suggestions welcome
Thanks
BD
BD,
Those voltages look ok from what I remember on the 6AN8. I don't have anything written down for the Sceptre's that I've worked on, but I do have something written down for my 200S, so I'll check that tomorrow and post if its different by a lot. The voltages are higher and the circuit is different on the 200S, but its close enough to where the voltages shouldn't be too far from that.
You're biased a little on the cold side for a 42 watt KT88...assuming the modern KT88 has the same reading as an old one, but I don't see why that would cause a problem with the trem...just making note of it.
Trem getting worse when NFB is disconnected....seems like it could be just because of the normal gain increase with the NFB loop disconnected.....but pulse with volume and trem turned down.....Maybe the roach is screwed up or the transistors are leaky or something. I'd check and verify all resistors around the trem and around the roach gain stage, and check the caps around that stage too. Something is letting that oscillation go and get amplified when it shouldn't. On the schematic I have, those caps around the oscillator are not shown as electrolytics....so maybe some cheap films or ceramics might work? Ceramics are often used in trems because they don't affect the sound and are more stable under certain circumstances too. Pretyy confusing but maybe if you take a step back and check some parts....analyze the circuit and how it could be doing what it is doing, then maybe you'll find the problem?
Greg
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Hi Greg
Go back a couple postings and check out the schematic I posted. It shows the caps in the trem curcuit having + symbols on the side of the caps . Hmmm I have a Sceptre schematic that doesn't show them vein polarized .... Not sure what to make of it .... I think I'm going to takethe board out again and desolder one end of every resistor and check the value . Maybe one is open ... I have to se if I have a transistor tester on my multimeter . I looked at Ratshack for transistors and had no luck...
What is a good bias setting for kt88's.
I really appreciate your help with this amp. I want to get it wright so I can enjoy it.
Thanks again Greg
You Rock
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Hi Greg
Go back a couple postings and check out the schematic I posted. It shows the caps in the trem curcuit having + symbols on the side of the caps . Hmmm I have a Sceptre schematic that doesn't show them vein polarized .... Not sure what to make of it .... I think I'm going to takethe board out again and desolder one end of every resistor and check the value . Maybe one is open ... I have to se if I have a transistor tester on my multimeter . I looked at Ratshack for transistors and had no luck...
What is a good bias setting for kt88's.
I really appreciate your help with this amp. I want to get it wright so I can enjoy it.
Thanks again Greg
You Rock
BD,
I saw a jpg you posted but not a schematic. I can see in the jpg the pic of the amp and the caps in question, and they do look like electrolytics, but there is no reason you have to use an electrolytic as long as the value and voltage rating is correct.
Ratshack is pretty useless these days and has been for like 15 years. We have a local store here that used to be pretty good but even they are getting bad because while they can get you anything you need for the most part, they aren't stocking much of anything anymore which really sucks when you need to visually see the part to see if it will work or not. As time goes on, places like Mouser and AES/CE are going to be where people go to look for stuff all the time instead of the local stores, because no one is stocking anything anymore.
Personally I don't like vintage tube Sunns much for guitar, but I do like them a lot for bass. But if you like it when you get it done, then thats what counts. For the KT88, the vintage spec was 42 watts dissipation, so if you figure 60% to 70% of that, you should have 25.2 to 29.4 watts dissipation. So if the plate voltage stayed at 537v, (which it will not as you adjust the bias hotter) then 50ma would be in the lower to middle end of that range.
Greg
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If I don't have to work a 12er tomorrow I'm going to camp my but in the basement And fix this dam amp. The truth is this amp is for my brother and he wants it back soon. He uses mostly pedals and just wants and amp witha nice cleans tone . I built him a bassman out of and old late 50's phonograph and it sounds pretty dam good but I think hd wants something that looks a little more like and amp than a robot on stage. I'll post my findings. If it's the trem bug it sounds Ike I'm screwed...
Thanks again Greg
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A Sunn is a good amp for use with pedals....other good ones are HiWatts and Vox AC100's and AC50's. I made myself a clone of an AC100 that is fantabulous, though it isn't done yet. Anyway, once you get it dialed in, I bet the Sunn will work fine. If the roach is bad, you can make another, but you need to know some specs of what is there so you can get the correct parts. I can see if Conrad has any ideas for you on this problem if I talk to him soon. If I get a chance to talk to him and he has anything to add, then I'll just tell him to post here BD.
Greg
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I made a matchless spitfire out of organ amplifier that sounded prety nice. I gave it to a friend of mine who was in a rut with his guitar playing . I had my brother make a head cabinet for it . Looked like a small marshall head but with less knobs. I have 8 or so organ amps sitting in my basement . Hmm
Do I see another spitfire in my future... Not a pedal amp but a nice well rounded amp with the master volume. You could pull off some nice country licks or get down and dirty . Oops rambling on again.
I went to ratshack and picked up some 1000uf caps for the filter section and a couple 10uf's . that's about all they had I could use. I have some 50v panasonic caps I might try for the 1 and 2nd caps . How do I know If the trem bud is bad?
Thanks again greg
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I made a matchless spitfire out of organ amplifier that sounded prety nice. I gave it to a friend of mine who was in a rut with his guitar playing . I had my brother make a head cabinet for it . Looked like a small marshall head but with less knobs. I have 8 or so organ amps sitting in my basement . Hmm
Do I see another spitfire in my future... Not a pedal amp but a nice well rounded amp with the master volume. You could pull off some nice country licks or get down and dirty . Oops rambling on again.
I went to ratshack and picked up some 1000uf caps for the filter section and a couple 10uf's . that's about all they had I could use. I have some 50v panasonic caps I might try for the 1 and 2nd caps . How do I know If the trem bud is bad?
Thanks again greg
I've got a bunch of organ amps and projects sitting around too. Not enough time to work on them with going to school for engineering, but have been working on a Bogen CHB100 that I gutted and modded for something I can use live at the jams. Its pretty cool and unique with a light up nameplate, a parallel input stage, 5879 pentode, and a quad of 7868's....58 watts RMS. It will be getting power scaling needless to say...haha.
I'm not sure how to check if the trem bud is bad honestly. Might do a search on the other areas on Ampage or on other sites like AX84, or Hoffman, or do a post and see if you get any help.
Greg
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I've had some pretty good scores as far as organs and stuff like that goes. I hooked up with a guy who owned some storage units. One of his tennents who was renting 6 storage units had died and his family didn't want anything to do with the stuff he had in storage. The units where full of organs , old stereos , clothing and prety much anything gun an imagine. The guy was a pack rat to say the least. I helped the guy clean some of the units and he gave me what I wanted out of them . I hit about a 5 gallon bucket full of tubes and one organ had around 20 telefunken 12ax7's inside . J was like a kid in a candy store . Anyone else would have seen junk but I saw treasure but know with shows like Storage Wars and Pawn Stars all those days are in the crapper. ..
http://a4.l3-images.myspacecdn.com/images02/20/a61f7329d7984d4daf63a45e4a32b7b0/l.jpg
http://a2.l3-images.myspacecdn.com/images02/93/d03630d4d51042d7a95b95ec98bfd9bd/l.jpg
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Hi, new member here and just thought I join the forum to chime in on this topic.
I too had a sceptre with a similar problem- clicking in the tremolo that would cut in and out with variable intesity depending on the rate and depth settings. I bought this amp after it had just been done over and some of the major stages had been recapped but not the tremolo circuit.
I found this post describing some similar problems- http://sunn.ampage.org/sdp/index.php/topic,2071
and so I decided on trying to fix it by replacing the 2 1000uf 35V electrolytics on the trem board.
Just finished up about 10 minutes ago and so far so good! I haven't really put it through it's paces yet, but it holds up ok with a p-bass through a little 2x12 with the volume at about 4/5. I usually run my sunn's (I also have a sonaro) pretty close to wide open when using them for bass, and right around 7/8 for guitar. I'll report back after a full run, but replacing these caps seems like a good fix for this kind of problem. Probably a good idea even if you haven't run into trouble with them yet...
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Hi, new member here and just thought I join the forum to chime in on this topic.
I too had a sceptre with a similar problem- clicking in the tremolo that would cut in and out with variable intesity depending on the rate and depth settings. I bought this amp after it had just been done over and some of the major stages had been recapped but not the tremolo circuit.
I found this post describing some similar problems- http://sunn.ampage.org/sdp/index.php/topic,2071
and so I decided on trying to fix it by replacing the 2 1000uf 35V electrolytics on the trem board.
Just finished up about 10 minutes ago and so far so good! I haven't really put it through it's paces yet, but it holds up ok with a p-bass through a little 2x12 with the volume at about 4/5. I usually run my sunn's (I also have a sonaro) pretty close to wide open when using them for bass, and right around 7/8 for guitar. I'll report back after a full run, but replacing these caps seems like a good fix for this kind of problem. Probably a good idea even if you haven't run into trouble with them yet...
Every electrolytic cap has been changed in my amp... I have a pulsing sound not a ticking sound.... The pulsing sound is there with the volume and trem on zero. I also get a little guitar signal with the volume at zero . I think i might have a bad trem module and might just disconnect or bypass the dam thing and say the hell with it. If i turn the trem up the pulse does speed up but I'm thinking I shouldnt hear anything unless a signal is applied. Im also doing my testing without the reverb tank hooked up but that shouldnt make a difference....
Thanks
BD
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Is there a footswitch to disconnect the trem circuit or not on this thing, or any jacks for it on the amp? If you don't see one, then you can just pull the oscillator tube and the noise should go away. If it doesn't then the problem is somewhere else. See if pulling the phase inverter tube makes it go away...if it does then you know for sure it is before it....see if you can use this approach or shorting out one grid at a time to see if you can localize where the problem is. It will make it much easier to solve BD.
Pretty cool pics on the organ amps...looked like you might have had a Conn in there? (2nd one in front on the right) If so they usually had really nice transformers...Foster often, Schumacher sometimes. Fosters are really good btw....made in Cinncinatti and the company is still around and in business, though I don't know if they are doing tube amp stuff anymore or not.
Greg
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I'm sure there was a conn or two in there some where . I usually get Hammond with a Baldwin every once and awhile . I also get some nice speakers as well like 15" , 12" and 10" jensens. I think those days are gone though.
I groundedout the footswitch and the problem was still There. I hate typing from my phone. I do some other checking tomorrow or I should say later today.
Thanks
Bd
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I tried shorting out the footswitch jack and the problem is still there. I pulled the phase inverter and the problem is gone. I pulled the 12au7 reverb driver tube? And problem is still there . I pulled the preamp tube and problem is still there. I pulled one tube at a time and put it back in and pulled the next.
I didn't mention it but i just had a baby girl October 14 of last year so that's why it's taking a while to fix this amp. I can only go in the basement and work on it in spurts. I'll try the shorting method tonight or tomorrow .
Thanks again
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I tried shorting out the footswitch jack and the problem is still there. I pulled the phase inverter and the problem is gone. I pulled the 12au7 reverb driver tube? And problem is still there . I pulled the preamp tube and problem is still there. I pulled one tube at a time and put it back in and pulled the next.
I didn't mention it but i just had a baby girl October 14 of last year so that's why it's taking a while to fix this amp. I can only go in the basement and work on it in spurts. I'll try the shorting method tonight or tomorrow .
Thanks again
Congrats on the baby girl BD! I understand completely.
So its before the phase inverter, but after the preamp tube then where the problem is, which could be the roach for the trem since it works after the first gain stage. I'm not sure how to check those roaches and don't know if the fender bug works or if you need another. I may have to talk to Conrad in the next couple days so I can ask him if he remembers....
Greg
Nevermind about asking Conrad...he posted in this post about how they made the roach.... http://sunn.ampage.org/sdp/index.php/topic,2071.0.html
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I might just bypass it by the volume pot and see what happens . If the problem disappears than I should be the trem module.
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I jumped the footswitch again and it did quiet the pulse but not eliminate it. Maybe the trem is getting amplified to ground and somehow getting Bach in the signal chain somehow.
Well I tried bypassing the trem at the volume pot and it didn't work ( still had the pulse ) so I disconnected the power from the board and pulled the 12au7 for the reverb. I'm thinking about ordering some transistors for the board.
The amp has some serious ground issues for sure. If i drag my fingernail across the chassis i can hear it in through the speakers. :x
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I took voltage readings with the 12au7 removed and the B+ jumped way up. Hmmmm I put the tube back in and the B+ went back down. Is this normal?
How much of a voltage drop should I get across the choke? It's a 60ohm choke.
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Still messin with the Sceptre. The voltages in the amp are to high even for the second filter stage ( 500v Cap ) to handle so I built a voltage browner with parts from Ratshack. IT drops the 120v ac to 112v ac . Here are my voltage readings with browner
112v ac
Bias 45ma @ 500v plate
6v ac heaters
Filter Stages
505v 1st
497v 2nd
398v 3rd
315v 4th
279v 5th
If my brother really likes the amp I might have him buy a V-Dump from Weber Speakers.
Trem is still pissing me of to no end. All I know is the trem board adds allot of noise to the amp. I mean allot of noise.
I did have the voltage increase problem again when I was in the basement messing with the amp. I have all of the tubes in and it jumped up like 100v across the whole amp. I'm talking 400v on the 5th filter stage. But I shut the amp off and try it again and the voltages are back to normal. This only happened one when I was down stairs today. I wonder if running the 2nd filter stage over 500v for a wile damaged the 2nd stage filter cap??
I also had a weird thing happen. I got a noise through the speaker that sounded like Morris Code. Hmmmm I had this happen once before when I was working on the am[p. I did have my cell phone close to the amp when I was working on it.
Thoughts welcome
BD
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It seems like this amp should be louder for being a 60watt amp. Hmmmmm Maybe my hearing is going or something... It sounds pretty decent and stays relatively clean dimed ... How can I determain the watts it putting out?
Thanks again
BD
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The voltage jumping around thing is probably an oscillation. I had that in a project amp in the past...I never did solve it on that amp as I decided to do a new seperate preamp chassis design and never got around to it. But the voltages in one section of the preamp would jump by 100v for no apparent reason. The first two cap sections should have at least a 600v rating...the third one is fine with 500v. That is why I series connect caps for the first and second section...because two 350v caps will give a 700v rating....if you do it, don't forget the parallel resistors with each cap or your caps will blow. Your wall voltage is pretty low there....112v....here it is usually 120-125v, which makes the AC voltages much more of course. My 1970 Sonic 1 (200S) is often at 560v B+, and a customer's Spectrum 2 was at 520v.
The Morse code sounding thing is probably your cell phone...I've had that happen too. Tube circuitry being high impedance is very sensitive to things like that.
As far as checking the output power.....do you have a scope, signal generator, and dummy load? If so then it is easy. You input a 1KHz signal about 200 mv (.2v) with the amp on a dummy load. Monitor the output with the scope and get the signal with the highest amplitude with the trace just barely clipping, and then back up from there until it doesn't clip anymore. Measure the AC RMS voltage at the output, square it, then divide by the impedance, or in this case resistance. (Helps to have a true RMS meter, but without that you can still get a relatively accurate idea) You can use a speaker instead of a dummy load but it isn't as accurate since it's impedance changes with frequency and a resistor's does not. The tone controls should be at zero also. Anyway, say you have 28v AC RMS and 8 ohms.....so 28 times 28 = 768/8 = 96 watts RMS. Most of the Sunns that used two KT88's or 6550's will be around 70 watts RMS and a peak power much louder....thats with NFB engaged. They tend to break up around 4-5 and the distortion character is harsh and hard compared to your typical Fender, but they have a TON of bottom and top end. Some people like them, others don't. I fall in the camp of I don't like Sunn tube amps much for guitar, but love them for bass.
Greg