Newbie quetion

[loudthud]

Here are two shots of the inside of the preamp. My amp doesn't have the octal plug to get power from the power amp, it's hard wired. The two output jacks are wired in parallel with leads going to the screw terminals on the power amp. It's wired with #22 white zip cord just like the speakers I had in the early 70's. Note: to get more than one picture per post, click on the blue text that says (more attachments).

[Jenkster]

Thanks loudthud, you're the best! Here is a picture of mine. As you can see, the "hack" who worked on it liked to take out caps and not replace them??? What is the voltage rating on the 1000 uf caps and how are the diodes connected, if those are in fact are diodes on top of the caps? There are other issues, YEAH! Maybe they tried to convert this to a PA amp? One input with two or four outputs to mains or monitors? I will probably never know.

[loudthud]

The two caps and two diodes are the voltage doubler circuit to power the heaters of the preamp tubes on 12V DC. On the schematic they are in the upper right hand corner. The 1000uF caps only need to be 10V or more. I see your preamp has twisted wires so the heaters must be running on 6.3V AC. Note that on the schematic, pin 9 of both tubes is grounded and there is no ground at the heater winding center tap, only a cap to ground (check this if you want to restore this circuit).

Switchcraft used to make a little passive mixer with 4 inputs and 4 volume controls which may have been what was mounted in the bottom of your amp. There is a story going around that Conrad used one on the first Beach Boy's PA. So maybe ...

There is another story that Elvis Presley used 5 Dynaco MK III's in his PA at one point. Nobody has said this in a long time, and most of the readers here aren't old enough to remember, (Conrad is a notable exception) but vacuum tube PA's sound really good if they are loud enough. And, ya gotta use JBL speakers!

[Jenkster]

That is probably why there were 12AT7's in it which are lower voltage, but why would you want to do that? 1000uf caps are pretty big for those days, I was guessing 25 volts or so. One curious thing I just noticed was the dates on the pots. The bass and treble pots are dated JAN/FEB 1966 respectively. The volume pot is dated JUNE 1966? Assuming it is original which it appears to be, isn't mid 66 kind of late for this amp if it was a bass head? I know I can "what if" this thing to death but could this be an early attempt at a PA amp???
Interesting about the Beach Boys and Elvis. Oh, 1958 D130's in my old bassman cab, re-machined for the larger voice coils and re-coned in Northridge, E-140 in a homemade single cab. My garage speakers for stereo are JBL Model 19's and 4301B Broadcast Monitors.

[loudthud]

I'm not sure about the date the switchover to front controls occured. There is a 66 date code on the 1000uF caps in my amp, the serial number is 14xx. ( I won't be able to look at the amp until Monday.) The DC heater supply is what was used in the Dynaco preamp at the time. I think Conrad made some changes to the preamp circuit vs the Dynaco but I've never compared the schematics side by side. Back in the early 70's I made a clone of one of these preamps and I remember having a hard time finding a 750K pot. There is not much difference in ratings between 12AX7's and 12AT7's except for the gain. The heater ratings are the same.

It's hard to speculate why things were done the way they were on your amp. My guess would be that who ever did it didn't have the money or access to parts that we take for granted today.

I have a smiley face 2x15 cabinet (kinda beat up) that has a pair of D130's with consecutive serial numbers. Too bad the speakers are blown. I wish I could find someone I could trust to re-cone them.

This thread has got me thinking about building an all tube PA. The system I had for years was a pair of Altec A7 cabs with D140's, biamped with a 20W per channel tube power amp running the horns.

[Jenkster]

Grumpy old Tom at Aural Technologies in Ashland, http://www.auraltek.com/ ,does a great job! He's done work for me and others  I know and haven't had any complaints. He is just sometimes kind of grumpy! If they are the older D130's, the hole for the voice coils will have to be enlarged to accept the new ones. Tom will send them to Northridge and they will also be re-magnitized. It will not be cheap however!!!

[Isaac]

OMG! Tom Garson is still around? We played together for a while, in a band called Echo. That was before he dyed his hair green to play with Johnny Green and the Greenmen. I don't think he liked me much.

[Jenkster]

I wouldn't feel bad Issac, I don't think Tom likes anybody too much! I found a discrepancy in the Dynaco Mark III schematic and that of the SUNN Model 60 schematic. The SUNN shows a 50 ohm, 5 watt resistor on the 30/20 can cap. The Dynaco shows a 6800 ohm resistor. It's probably not a huge deal but that is about the only deviation I can find between the two and I was wondering if the SUNN schematic is correct?

[Soundmasterg]

I can't say if that particular Sunn schematic is correct, but the early 200S uses a .1uf coupling cap between the PI and the power tubes, and the schematic shows a .25uf, which is what is in the later 200S. There are also voltage differences with the early amp being around 470v B+ and the later one more like 560v B+. So my guess is that there were running changes on everything including your Model 60.

Greg

[Isaac]

Tom did make me famous, though. Well, somewhat famous, on a regional level. Every time I'd do, well, pretty much anything, he'd say, "Fuckin' Isaac" or "Fuckin' Isaac, man." He said it so often that it made its way into the local language. Our singer was visiting a friend in Roseburg. The friend dropped something in the kitchen and said, "Fuckin' Isaac!"

Sally replied, "What did you say?"

"Fuckin' Isaac. Everybody says it. No one knows where it came from."

One thing about Tom is that he was an amazing guitar player. Amazing not because he's good, although he is very good, but because all of his left hand fingertips have been cut off. Just the first joint of the index finger, but the second joint on the others. Combine that with the fact that he's quite short, is it any wonder that we caled him Tom Thumb? And then, when he dyed his hair green, how could we not think of leprechauns?

[loudthud]

Back to the Model 60. I examined my amp against the schematic and there are differences.

The input to the power amp has a resistor to ground marked 1 meg and 470K. Mine is a 40 watt and it has the 470K. A MKIII Dynaco I have also has the 470K.

The schematic shows the first 7025 with a 1K cathode resistor. My preamp has a 2.2K there.

The schematic shows both 7025's with pin 9 grounded. On mine, only the first 7025's pin 9 is grounded.

The schematic shows an extra 10uF cap on the middle B+ node in the preamp. My amp does not have this. It has a 4 section 10uF @ 450V. The incomming B+ has two sections in parallel and one cap per node with 10K decoupling resistors.

The common side of the output transformer is grounded.

FYI the 1000uF caps are rated at 12V.

[Jenkster]

Thanks Loudthud, Mine does have the 1K resistor on V1a cathode. Another question, do the Bass/Treble boost make much difference? Mine don't seem to make any difference. Do you have a pic of the inside of the power amp?
                     

[loudthud]

My amp is a 40 watt so it has a Mk IV power amp. I got it on ebay and the power amp broke loose during shipping . Thats another story. Anyway, I've never turned that amp on since I got it so I can't comment on the tone controls. They might work better with a 12AX7 since they are in a feedback loop. The MK III power amp I have had some serious mods when I got it so I don't think pictures will be of any use to you. I did notice it looks like your transformers have been re-painted.

[Jenkster]

Thanks, I also have the 470k and no 1meg in the power amp. I have to believe the boosts should make a difference so perhaps I'll try replacing them just to see what that does. Thanks again.