Restoring 6v6 PP Amp from 1960 Voice of Music 915 Console

danneskjold

New Member
Hello Everybody,

I'm attempting to restore my great-uncle's 915 Voice of Music console, this thread will focus on the 6v6 PP amplifier, and maybe branch off into the pre-amp as well. Visually the 915 is identical to the 815 shown on this page; the only difference seems to be the addition to an AM/FM receiver in the 915.

The console was the first thing my great-aunt and uncle purchased after they got married; adjusted for 2013 dollars it cost over 3K and was moved from place to place until my great-uncle gave it to me last week.

I have no previous experience with antique audio equipment, but I follow directions well =)

Progress log:
12/25/12-Get console home, blow out dust, contact Gary Stork at voiceofmusic.com for manuals.
1/4/13-Pull amplifier from console, remove and store the tubes, start to figure out what caps to order
2/10/13-Replaced filter and signal caps, unit will not power on
4/15/13-Purchased identical amplifier and pre-amp for troubleshooting
5/15/13-Replaced filter caps in second amp, second amp and pre-amp power on, unit sounds "ok"

Resources:
Schematics of the 6v6 power amplifier from the VoM 812, 912 & 815 consoles


Schematic of the Pre-amp that came with VoM 912, 1001 & 1002

Question #1-Schematics show the image below for capacitors, I understand I will be replacing the filter (can) cap with 4 individual caps on the underside of the chassis, but for C3, c4, c5, c6 what voltage should i be looking for?

Question #2-I understand there are different types of caps, and brands as well; what types and brands should I use?





Best,
Danneskjold
 
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i restored two VOM amps. one i used Jantzen caps, thetas, etc.. what a waste of money. i recommend Nichicon, panasonic or Dayton caps. i used 100v values for the caps you are questioning. Great little amps!
 
Question 1: Caps 3, 4, 5, and 6 should be replaced with caps that are 400 volts or better.
Question 2: For the large filter caps I'd suggest either Nichinon or Panasonic. I also try to get the ones that are 105 degree celsius with low ESR: Shouldn't be any more than 4 or 5 bucks a piece.
As for the coupling caps, you know, C3-C6 I would suggest 716p Orange drops.
There are plenty of people here that would suggest more expensive coupling caps but for the money 716p ODs are hard to beat.

No offense to the previous poster, but I have never once seen a PP amplifier with 100volt coupling caps, period. 400 to 600 volt caps are the general rule.

Good luck with the resto.
I have the el84 version of your amp and really, really like it!!
 
Thanks Vinyl and Pilot. I found Orange Drop 716p at www.tubesandmore.com in the correct voltage and mfd-will order them tonight.

I can't seem to find nichinon or Panasonic in the correct mfd and voltage on that site, or digikey, or mauser.

Can anybody suggest sources, or if I can substitute different mfd or voltage?

Best,
Danneskjold
 
Hey again!
I'm a big fan of Mouser. They should have the caps you need.
Try typing in 47uf @ 450v and see what you get.
As for the 60uf cap, I replaced mine with two 33uf @ 450v caps wired in parallel to give you 66uf. Keep in mind that with the filter caps you can go a bit over on the uf but not under.
When all was said and done, I used 1 47uf @ 450v, 2 33uf @450v, and 2 22uf @450v.
Just checked Mouser for shits and giggles and all the above caps are in stock.
Good luck!!!
 
Thanks Very Much Pilot!

I ordered the caps a few weeks back, and got around to finishing the soldering job today. Put the tubes back in, put the amplifier back in the cabinet, plug the tuner power socket and the pre-amp power socket back in. Plug the power cord into the wall, turn the selector knob from "off" to radio and...

Nothing. Nada. No smoke, sparks, sound, warm glow from the tubes, or anything. I don't know when this unit worked for the last time, same symptoms even if the selector knob is on tape, radio, phono, etc.

Questions:

1. Am I looking for an issue in the amplifier itself, or could it be elsewhere in the cabinet?


2. Assuming my new caps are correctly installed, where should I begin the trouble-shooting process?

3. I know that I am supposed to unplug the amp and short the filter caps by placing a jumper/screw driver across their leads before working on the unit, do I need to take any other safety measures while this thing is on the work bench?

Thanks to all for the advice, this has been fun so far.

Best,
Danneskjold
 
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Is there a fuse?

Another thing, the power switch could be dead too.

You have to verify that voltage goes in the power transformer and also goes out...

You have the schematic?

Patrice
 
Get yourself a DVM (Digital Volt Meter) and take voltage readings from the power cord, primary of the power transformer, and secondary of the power transformer with it all connected up & turned on. Of course, follow safety procedures!
 
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Question 1: Caps 3, 4, 5, and 6 should be replaced with caps that are 400 volts or better.
Question 2: For the large filter caps I'd suggest either Nichinon or Panasonic. I also try to get the ones that are 105 degree celsius with low ESR: Shouldn't be any more than 4 or 5 bucks a piece.
As for the coupling caps, you know, C3-C6 I would suggest 716p Orange drops.
There are plenty of people here that would suggest more expensive coupling caps but for the money 716p ODs are hard to beat.

No offense to the previous poster, but I have never once seen a PP amplifier with 100volt coupling caps, period. 400 to 600 volt caps are the general rule.

Good luck with the resto.
I have the el84 version of your amp and really, really like it!!

Exactly right, but I have, only in my own gear, used some 200v coupling caps. If you actually measure the voltage that the cap will see it is always much less than the caps rating. Only if a resistor shorted closed would voltages that high be seen across the cap. If you have an amp open, try measuring across the the cap (this is the only voltage the cap actually sees). I only do it if I have some Holy Grail caps but only in 200volt ratings. I must say though, that I've never lost a single one. Westons, and Vit.Q or Hyrell mostly.
 
Hi Patrice,

I see no fuse in the schematics or cabinet (but may not know what I'm looking for).

Schematics are here.

Next weekend I'll check the power cord and primary/secondary windings on the power transformer.

Thanks all!

Danneskjold
 
May updated

Hi All,

Quick update-

I tested the power cord and attempted to test the transformer based on the schematic. Several of the wires coming off the transformer didn't have the voltage recommended, and based on that I believed the transformer was dead.

AFter purchasing an identical (except for the lack of a radio section) amplifier and pre-amp from ebay, I replaced the filter caps in the second unit and did a test last night. No love until I swapped in the second pre-amp, and then success!

So it seems that in the original unit the pre-amp was the culprit, and not the amp itself. Next steps-replace line caps in the second amplifier.

Notes-RCA standards have apparently changed since 1960! My newer RCA jacks have all kinds of shielding and rubber around the plug-the female connectors on the amp and pre amps are too close together for the new style to fit. Also, the "male" part of the male plugs is longer-may have been causing static the first time I powered the unit up.

The unit sounds big, and bassy, no lack of volume, but not high-fidelity by a long shot. It doesn't sound very good =)

Question: Any advice on eliminating the tone controls? In my mind it will simplify the circuit, and I can do any adjustments better through (gasp!) the computer that will eventually provide the source.

Partial Answer: Here is a thread by Rev man where he asks a similar question, but the answer is too complex for me to follow. It seems getting rid of the tone controls is desirable (and he had good success getting rid of the "tone-o-matic" volume control as well).
 
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You can get small RCA plugs to fit those old jacks. Modern interconnects do tend to be fatter by a fair margin. I'm using some basic metal Switchcraft plugs on my various old gear. I think they came from Digikey, but I won't swear to that.

Do you have a schematic for the preamp? It may be fairly simple to bypass the tone controls but depending on how it was done, you may have to make some other changes to the circuit to reduce gain. Usually thats the case with passive tone controls. They work by cutting a large amount of signal, and if just jump it out you'll have screamingly loud volume with a very tiny amount of signal input.

Also, are you using this with the original preamp or no? The feedback circuit appears to run into the preamp. If thats not present, you'll have no feedback loop to help promote flat frequency response. What speakers are being used? Some console speakers just weren't that good. Boomy bassy sounds pretty much like a lot of them were, especially the ones with no tweeter to bring out the highs.
 
Thanks for replying Gadget!

Here is the schematic for the pre-amp.

It seems like taking out the tone controls on lots of old amps is desirable, just not sure what to cut/solder to get that done.

I am currently using the original pre-amp, and do understand that I have to maintain the feedback loop. Yes, the speakers are original 12" and tweeters w/ some sort of crossover. The speakers will likely be replaced at some point, but want to get the pre-amp/aplifier into reasonable condition first.

best,
Danneskjold!
 
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If I'm not too far off here, I think you can bypass the tone controls fairly easily in this. Break the connection between C7 & R7 and the grid of V1. Unhook all of the stuff from the grid of the second half of V1. Connect C7/R7 to the second grid of V1. Other channel is the same idea, just with the appropriate part numbers. That should feed the volume control and input selector straight into the voltage amp and retain the feedback circuit without the tone controls in the way.
 
Hi Gadget,

Thanks for your help. I modified the original diagram to reflect what i think you're telling me.

Am I on track here?

Trying to measure twice, cut once :D

best,
Danneskjold
 

Attachments

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Weekend Bump-

Can anybody check that schematic and point out my errors?

I'm sure there are some, but don't want to find out after I cut and solder!

Best,
Dan
 
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