Trutone Stereo amp

[angelodp]

Pics as of now

[sluckey]

but the V1 heater bias is at 8.2VDC... tad low.

Is this with all four output tubes? If so, this is an indicator that the output tubes are weak. 8.2v is not enough to properly heat V1 filaments. I strongly suggest wiring V1 heater to the 6.3vac filament string. Then put a proper cathode resistor/cap on the cathode of the output tubes. I would split the output tubes into their respective pairs and use a separate 130Ω/3W and 100µF@50v on each cathode pair.

Could you post the actual schematic file or a link to it on the web rather than your screen capture?

[angelodp]

https://products.electrovoice.com/binary/2225%20EDS.pdf

Yes with all four tubes.

Split the pairs.... etc? You make it sound easy... I'll try all that.

[angelodp]

Any harm in undoing that NFB loop on both amps? 68k/50pf? That might get some gain going?


BTW I do have some EL 84's on hand?? Conversion thoughts.

[angelodp]

Ok pulling that NFB has upped the girth and volume and gain for sure. I disconnected the leads from the 16ohm tap and it has induced some hum. I need to clear all those wires and components off the pins and at the OT.

Any issue with this.... no NFB?


I re added a 240k R with no cap..... marked increase in vol. Still not quite guitar amp tone.

Maybe the EL84 route might be the way (as I have a bunch on hand). Not sure if this PT would handle -2 EL 84's

Maybe go towards a Liverpool?

[sluckey]

This amp is not a good candidate for EL84 conversion. However, you could easily convert to the cheaper and more available 6BM8. Same kind of tube but different pinout.

I've put together some drawings showing some of my favorite preamps. See attached pdf...

[angelodp]

Any problem using 100uf @ 450v for the Cathode bias? I have them on hand?

A

Ok I tried your prompts Steve, now cathode biased with splitting of the power tubes and standard heaters to all the preamp tubes. It works, with somewhat higher vol. level( so improvement). I suppose this is a better arrangement. I am looking at some ECL86 at a pretty good price. Take a few days to see if I want to split this down to a single amp and go with the power tubes you recommended and adjust the preamp.

Thanks

PS the cathode arrangement is a tad oversized and temp.

[bodhi]

I don't have anything to add to the actual discussion, but just wanted to chip in that the PCL86 tube to my understanding (based on some online reading, FWIW) is pretty similar to the ECL86, only it uses 12V heaters... So that could be an option if you could fit a separate transformer for heaters at that voltage, and the tubes are bound to be cheaper...

[angelodp]

Thanks for the suggestion. I have been cleaning things up a bit. I restructured the speaker jacks to the chassis for 8ohm & 16ohm (speaker cabs I have readily available). Steve I like the two pre-amp ideas.

Here is my current thinking, also with input from original owner.... he would like to still have a stereo setup.

I have a radial ABY box (Big Shot), with that box one can feed two amps and or a single amp. The idea would be to have two guitar inputs, one for each of the amps (pre-amp & power), with outputs to separate speakers.

Or, I could still implement the balance idea between the two amps with a single signal input.

Owner has a few extra ECL86's to play with.

Am I correct that the triode side of the ECL86's is serving only as phase inverters in this design.

I am still holding onto the third 12ax7, and might use one side as a boost/gain to the circuit on a switch.

[Phil_S]

Am I correct that the triode side of the ECL86's is serving only as phase inverters in this design.

The ECL86's are V4-V7. V4A and V6A look like gain stages to me. V5A and V7A are concertina PI's. The answer to your question is either "no" or "only two of them."

[angelodp]

Thanks Phil, so if I intend to follow the schematic that Steve put forward, do I include the triode gain stage in the ECL86 as shown or jump past it? Is that ECL86 triode stage serve as recovery from the tone stack?

[sluckey]

You must connect those preamps to the grid of V6A. I thought that was perfectly clear in my schematics? You also need to add that 1M to the grid of V6A.

The concertina phase inverter has a gain of slightly less than one. Many people consider the triode just prior to the concertina as being part of the PI. That's what I did on the schematic.

[angelodp]

Thank you for the clarification.

[angelodp]

Question, on the speaker jack arrangement, I have gone to two witchcraft, one for 8ohm and one for 16ohm.

In the original setup the NFB wires both went to a 16ohm tap. I think I have it wrong now the NFB on the 8ohm tap (thin yellow wire) should go over to the bundled wires, yellow cloth (16ohm ) from that OT and not to the
blue cloth 8ohm lead at the jack?

[Phil_S]

Yes, it looks like yellow is the 16Ω.