Traynor YMV-1 Voicemaster to Guitar conversion - diyAudio
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Old 10th October 2009, 04:14 AM   #1
TheGimp is offline TheGimp  United States
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Default Traynor YMV-1 Voicemaster to Guitar conversion

A friend of mine has a (Yorkville) Traynor YVM-1 Voicemaster that he has asked me to modify for guitar inputs.

Not a big deal I thought looking at the schematic:

http://www.lynx.bc.ca/~jc/yvm-1.jpg

First thing I did was replace all the Al Electrolytic caps with new 500V rated ones in place of the old 450V ones.

I change one input from capacitive to resistive 68K to grid/1M grid to ground, and plan on swapping the tone controls for a tone stack ala late 60s Fender designs. I later noticed that there was no cathode resistor in the input triode circuit (grid leak bias?) and tried using a 1K5 resistor with 22uF bypass. Better gain, but no real change in output compared to the un-modified input.

I changed two channels to a fender resistive input and he came by this evening to try it. The tone was much better, however the output was way low.

After he left I got to looking and found everything looked good up to the screen drive of the finals where I had 130Vrms at the screens of a pair of EL-34s.

However, I only had 33Vrms at the plates!

The plate swing seems WAY LOW. I was expecting near 300Vrms.

B+ is 449V
Screen supply is 436V
Grid is at -35.8V

I tried a second set of tubes (his original 6CA7s) and got nearly the same results.

The amp uses fixed bias, and I considered changing to cathode bias to avoid line voltage variation effecting bias conditions, but haven't made any changes yet. However I don't think this is really the problem. Everything looks like it should really drive the speaker, but it doesn't.

Suggestions?

Thanks.

Steven
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Old 10th October 2009, 06:01 PM   #2
TheGimp is offline TheGimp  United States
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The output tubes are drawing 61mA and 62mA of plate current at idle (inserted 1 Ohm resistors in series with the plates). When driven to clipping the dc plate current goes way up (200mA+) which of course is NOT good for the plates.

Even with no input, the plates would occasionally start to glow! Hit the Standby switch quick!

So I'm suspecting the output transformer may be bad.

I got to poking around and taking measurements and realized that I had the speaker plugged into jack 2 (of 4). Plugged it in Jack 1, and everything is great!

What The Hey?

Like many manufacturers, Yorkville went the cheap route. They only connected the ground terminal to the first output jack, and first of four input jacks, and relied on the mechanical mount to make the ground connection on the remaining three jacks for input and three jacks for output. Corrosion made for high resistance on all jacks, but the ones with the ground wires were not effected. With no good connection for the speaker, the tubes saw improper loading impedance.

Time to upgrade the wiring.
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Old 17th October 2009, 01:22 AM   #3
TheGimp is offline TheGimp  United States
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I replaced a bunch of resistors today, and got the amp back up and running.

As a test to burn it in, I hooked up an old 52X cd player which had the front control panel buttons for play/pause, next track, etc.

At first I was using a cheap (wally land) car speaker, and it was pretty tinny.

So, I dragged out one of my Kenwood KL-5050s and a Yes Fragile cd.

The intro to Roundabout is FANTASITC!

I think I need to add tube amps to the CD player (1AG4s), but I need to get to finish this guitar amp conversion and build my PP amp first.
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