just how crunchy should a 100s sound?

[aussie-sunn]

Hi!

I'm new to the sunn world, having just picked up a 1967(i think) 100s non-reverb head whilst visiting Sydney last week (up from Melbourne).

The amp looks to be a close cousin of the one featured in the Sunn Museum... Mine is serial number 2741 (12 behind the one pictured). It is in resonably original condition, but has a new power tranny (thanks to a 240V conversion, I'm guessing).

It is currently has Sovtek KT88s in it and was supposedly serviced before I bought it. It is indeed pretty quiet (only a little crackle from the tone controls) and sounds great up till about 2 1/2 on the volume knob. above that, it gets really crunchy really quickly (a lot more than a nice, bluesy kind of crunch).

I've tried playing it through a single 8ohm 12" and a single 4ohm 15" (using the centre-most speaker jack for 8 ohms and the outermost for 4 ohms - not sure if this is correct because the sticker is gone and I haven't had time to open it up and see which OT taps are connected to which jack) and experienced the same thing in both cases.

It's just not producing anywhere near the volume that I understand these amps are legendary for... ie: it's nowhere near as loud as my bassman 100 (and yes, I know it's less powerful than the bassman - but there's a big discrepency), although sounding much sweeter and richer at lower volumes.

I'm guessing that there's something wrong with the amp because of how quickly the its sound deteriorates - but on the otherhand, am still amazed at how good it sounds when clean... (which is why I am a bit confused about whether there is actually something wrong or not).

Also love how flexible the tone controls are (especially the contour control)...

Hopefully someone can educate me on these amps a little bit...BTW, I'm not afraid to go poking around inside the thing (I have repaired a few Fenders), although I'd appreciate it if someone could tell the best/easiest way to discharge the power supply caps (or if it's actually necessary viz. the fender).

Thanks in advance,
-matt.
 

[Fred Cook]

Something's wrong. The beast should strip paint and you should be complaining that you can't get it to distort!

Patience - Some of the forum's gearheads should be checking in with advice. In the meantime, I would definitely start cultivating the best tube amp tech I could find.

(BTW - beloved spouse graduated from Sidney Girl's High, not bad for a pom.)

[Bill Matthews]

It sounds like the bias is WAY out of whack.  On the wiper of the bias pot, you should get -55VDC.  If you have any more than that, you are going to overdrive the power tubes, and get that type of distortion you described. If you would like to discuss this problem at length, I will be happy to go thru the symptoms with you via e-mail.  Good Luck... Bill (vikkinbill@aol.com)

[Rocketboy]

After you verify the grid bias voltage is within spec, check the GZ34 rectifier tube.  Consider replacing it with a solid-state type (depending upon your 'druthers).  Also it would be a good idea to replace all electrolytic capacitors on a unit that old.  The 100S should blow the Bassman 100 straight back to the CBS studios.  If you haven't already done so, get the schematic diagram for your 100S on this website.  The circuit is truly elegant, and, armed with knowledge, you should have few problems.
Ric

[Rocketboy]

Sorry - Yes you need to discharge the filters before working on the amp.  I believe the procedure is to diconnect the mains with the amp off; then with the standby switch in the "on" position, jumper either plate of the preamp tube to ground for a few minutes; then jumper the plate of one of the output tubes to ground. That should do it gracefully.  Ric

[Anonymous]

Sorry - Yes you need to discharge the filters before working on the amp.  I believe the procedure is to diconnect the mains with the amp off; then with the standby switch in the "on" position, jumper either plate of the preamp tube to ground for a few minutes; then jumper the plate of one of the output tubes to ground. That should do it gracefully.  Ric

[aussie-sunn]

Hi all.

Thanks for the responses. I will check out both the biasing and the rectifier tube as soon as I can. When I look at the amp, the output tubes look new (sovtek), and the rectifier looks quite old (the glass is darkened, etc.).

It'll take me a while to work on it (so don't think I've given up if you don't hear from me in a while) because I'm pretty busy with work and the bassman is currently lying gutted on my desk - got a little ambitious at band practice last week and it has developed a wicked 50 cycle hum which is there regardless of whether there is any input signal present or volume position - comes and goes with the standby switch. Oh well.

Also, does anyone know where I can purchase original looking cloth to recover the front panel. It has been replaced with some blue stuff at some stage, and even though it looks kind of funky, I would really like to go back to the original silver colour.

Thanks again,
-Matt.

[Isaac]

Try Parts Is Parts http://www.guitar-parts.com

[aussie-sunn]

Hi.

Took a look around inside the amp yesterday...Very different in construction to the Fenders that I am familiar with.

Anyway, I think that this amp is one sick puppy. It looks like the 240V conversion has been severly botched. I was only seeing -26.7VDC on the wiper of the bias pot and this was with it adjusted all the way. At this point, I realised something was seriously wrong and took the following voltage measurements:

STBY SW: 379VDC
A: 356VDC
B: 254VDC
C: 254VDC (not too sure I measured the correct voltage here...will have to verify)
D (bias): -26.7VDC

These are obviously way out of whack...I guess the fact that they are so low explains why the plates weren't glowing red hot with only -26V at D.

I will confess that I'm a little lost at this point. Maybe it is the recitfier tube, although I'm not sure how to test it. I did pull it out and check for internal shorts and it looks ok.

Anyway, there are a number of things that I am not happy about, such as the fact that other caps have been added in place of sections of the quad-capacitor, yet one section is still in use. I haven't been able to find an exact replacement for this unit. The closest I've come is a 20/20/20/40mfd rather than the 20/20/20/30mfd originally spec'd. Does anyone think this will pose a problem? (I've read that it's no good to greatly increase the values of the filter caps in tube-rectified amps as the peak currents generated put too much strain on the rectifier) There is also a lot of bad soldering in there. ;-(

If it is the rectifier tube that is (part of) the problem, I am considering replacing it with a WeberVST CopperCap Rectifier solid state unit. According to the blurb "Each model emulates the forward conduction resistance curve of the tube type it replaces and also has linear in-rush current limiting to emulate the warm-up time of a tube rectifier." They are also a lot cheaper, at AUD$30... Does anyone have any experience with these?

Regards,
-Matt.

[Greg Z]

Copper caps have had good reviews,although
I haven't tried them, lot cheaper to stick in some 4007's. Just for s+g whats the filament voltage reading? And the lower the bias voltage the hotter the tubes will run.
Did you check the coupling caps for leakage?
the 20-20-20-40 will be no problem a gz-34 should handle that no problem, just make sure the caps rated 525v or more.