Trutone Stereo amp

[angelodp]

Thanks for eyeing that Phil.

As of now I have not wired up the preamp except for the heaters, which do have 6.6vac on them. I checked out the amp on bulb limiter without tubes, no issues, then loaded just the power tubes to look at voltages. At first the amp was quiet and bulb was not lit, then a hum built up and the bulb started to glow and glow. I shut it down. I do not have an input wired in or the 1M to the PI. I wanted to check voltages in the power section before proceeding . I will add the input with 1M and 68k, as well as the 1M to ground at the PI..... or should I just go for the two preamp stages before testing again? My plan is for two separate inputs - Gemini and Fender amps in one shell. I want to use the radial big shot any to combine the two amps.

[sluckey]

FIX THE HUM ISSUE BEFORE ADDING PREAMPS.

[Phil_S]

At first the amp was quiet and bulb was not lit, then a hum built up and the bulb started to glow and glow. I shut it down.

Ick. Sorry to hear this. Please do what Steve said. See if the bulb glows bright with no tubes. That, IMO, is a sure indication of a short.

[angelodp]

Without tubes the bulb limiter does not glow, it is very very slightly lit, I mean slight, but with speakers connected no hum and no light in limiter.

Back to your earlier notion Steve, these tubes are toast.

[angelodp]

I did add the 1M resistors at the grid of the ECL86 as per Steve's schematic and hum is dramatically reduced. When I look at just two tubes (a pair per each OT), its even quieter and lower glow. With the quad the glow and hum do increase but it far less than previously. I am going to re-introduce the NFB of the original schematic with was 68K with a parallel 50pf cap. I had gone to 240k to decrease the NFB.

The amp is open so some degree of hum is ok I believe. I am going to list my voltage per each side of the amp (pairs of ECL86).

[angelodp]

Question about operation.

I have the 1M R's in on the grids of the PI

I went to a 150w bulb. When I put in just two of the tubes on one of the amps, I get low hum and no appreciable glow. I switch those two tubes to the other amp and same outcome. I tried the same thing with the other two tubes I have and same result. It seems that running either side independently is fine. I think the low hum is from my dimmers in the room and the fact that the amp is open.

When I load in all four tubes the limiter comes on at a low glow but after a minute I do get a hum that gets
going a fair bit louder than previously. Could there be an oscillation generated between the tow amps now? I still have no preamp wired in.

[Phil_S]

Without tubes the bulb limiter does not glow, it is very very slightly lit, I mean slight, but with speakers connected no hum and no light in limiter.

Without tubes, there is no current being drawn. This condition is normal.

The amp is open so some degree of hum is ok I believe. I am going to list my voltage per each side of the amp (pairs of ECL86).

Open amp can pickup some hum, you cite the dimmer switches (below.) Agree this is normal. Yes, let's have a voltage chart!

I went to a 150w bulb.

IMO, no point in going above 100W. That is about equivalent, really a bit extra there, to the amp at full tilt. If it is OK with 100W, I think you are done using the limiter unless you make changes that cause you to want to use the limiter again.

When I put in just two of the tubes on one of the amps, I get low hum and no appreciable glow. I switch those two tubes to the other amp and same outcome. I tried the same thing with the other two tubes I have and same result. It seems that running either side independently is fine.

This is good! See prior comment about 100W. I think the amp is OK!

I think the low hum is from my dimmers in the room and the fact that the amp is open.

So, turn off the lights and see if the hum goes away! Turn off any in nearby rooms, too.

When I load in all four tubes the limiter comes on at a low glow...

See above. I think you can lose the limiter.

... but after a minute I do get a hum that gets going a fair bit louder than previously.

Not sure. You have 2x the amplification with 4 tubes. It might bring it over the tipping point. Maybe some sort of unwanted coupling between the two OT's? I don't know. I assume you do have a speaker load on both OT's? Remember running it without speaker loads can damage an OT.

Could there be an oscillation generated between the tow amps now? I still have no preamp wired in.

This is beyond my comfort zone. Anyone?

[angelodp]

OK, finally... have the Fender tone stack in and with 2-ERCL86 and 1 12AX7 in V1A, we have a rocker. I do have some low level hum with the amp open. I did not have a 250pf I trusted for the tone-stack so I put a temp 680pf in there. Tons of volume and gain, the tone-stack is quite active.

Moving forward, I need to see about the dual amp arrangement and see if feeding the guitar signal through the Radial box into both amps is gonna work out. I also want to see about a RAW knob and maybe a NFB switch.

Phil & Steve, thanks for hanging in and helping out.

I will get some voltage reading and check dissipation.

best A

[angelodp]

here is where I landed

[sluckey]

Since you are building two preamps you may be interested in this 3 position toggle switch that allows you to select either preamp or both preamps...

http://sluckeyamps.com/phoenix/phoenix.pdf

[angelodp]

Thanks Steve, I am going to redo my tone-stack on a board in the amp. No need for 300+ volts on the front board. I think two pre-amps into one amp makes sense. Having two power amps in a small shell seems
like asking for trouble, now that the gain is way up.

[Phil_S]

I do have some low level hum with the amp open.

For this, I keep a piece of cardboard to which I glued a sheet of ordinary (kitchen) heavy duty aluminum foil. It cost almost nothing. Do this. Put it on top of the open chassis. It is a good way to test whether the open chassis is the issue. Maybe not 100% conclusive, but still a very good diagnostic tool.

Congrats on the progress. It sounds like you are well on your way with this project!

[angelodp]

Hi Phil yes making progress. I am beginning to get excited about the sound. I have some work to do in getting lead dress better organized and a small board for the Tone stack components. Steve's switch idea is on the menu as well. Appreciate you help and interest in this little project.

[angelodp]

On the Gemini schematic, those .1uf bypass caps (on the cathode), they are not electrolytic .... correct?

A

[sluckey]

On the Gemini schematic, those .1uf bypass caps (on the cathode), they are not electrolytic .... correct?

Correct. You would have a hard time finding a .1µF electrolytic.