Trutone Stereo amp

[angelodp]

Hi, I have a Trutone Teeco 2225, I'll include the schematic. A few questions if you can help.

The unit was red plating the power tubes ECL86. I see some burned areas around the filament wires, I have replaced and re-flowed all those areas. The Electrolytic's are in process of being replaced.

I am new to the ECL 86 tubes. I do not quite understand the bias setup for this amp. I see the V1A &B filaments on the Cathode string of the Power tubes. Is that how bias is achieved? Is it DC?

On V1A I see that pin 4 is jumped to ground and not connected to pin 9.... that seems wrong, but I am not sure. Pin 9 has solder on it and is close the ground point on the chassis that the jumper goes to, but there is no connection to pin 9 of V1

Pin 5 of V1 goes over the the balance pot center eyelet and has the 100uf @25v cap that continues to the first ECL86.

I would like to better understand how bias is achieved on this amp and if the connections to V1 seem correct.

Thanks A

[sluckey]

V1 filament is a resistor. And this resistor is the bias resistor for the output tubes. This also means that V1 filaments are DC heated.

[angelodp]

Thanks Steve, is it that the juncture of the 100uf cap // to the filaments connects to pin 7 on one end and pin 4 of the V1 12ax7 on the other end?

I find this confusing. Can you clarify?

[sluckey]

A pic is worth *******************

[angelodp]

Ok makes sense, thanks. I take it that pin9 of V1 has no connection?

A

[angelodp]

Amp is up and running but..... without tubes the voltages are a tad high but normal, with tubes the B+ dives from 325VCD to 179VDC and Heaters drop from 6.2 VAC to just under 5VAC. I ran some tests on both PT and OT and they seem fine. The ECL 86 reportedly (by owner) had red-plated... for how long I do not know. The three 12ax7's work fine in a champ. Any suggestions of moving forward appreciated.

Amp sound is ok but vol is not very high with it dimed.

A

[sluckey]

Pull the output tubes and check for zero volts on grid of each output tube. If zero volts, replace the output tubes.

[angelodp]

Sorry I don't follow. Pull the output tubes (ECL86), then measure the grids of the power tubes? Leave the preamp tubes in place?

[sluckey]

Did I mention preamp tubes? I don't know how to state it any simpler. Maybe I should add, check the grid voltage on the empty tube socket, not the actual tube lying on the bench.

I want to see if any of those .047 coupling caps are leaky. If all the grid voltages are zero then the coupling caps are not leaking. So next logical thing to do is replace the tubes.

[angelodp]

Ok I get it. Looking at it

Thanks

[angelodp]

Ok 0volts on grids. .00006 is the most I see.

So is it that the red plating has stripped the tubes a bit and thus they are pulling down the voltage as the
current rises? I mean as it falls.

[angelodp]

So ECL86 are a bit pricey and I am not sure this amp could ever serve as a guitar amp. I am using the aux connection and now have an IEC socket and a switchcraft 12A with 1M as the input. Its pretty meh.

Steve, with the aux input I am loosing any of the previous gain stages, you think this is worth the trouble to re-route the signal and fuss with the circuit?

[sluckey]

So ECL86 are a bit pricey and I am not sure this amp could ever serve as a guitar amp. I am using the aux connection and now have an IEC socket and a switchcraft 12A with 1M as the input. Its pretty meh.

Steve, with the aux input I am loosing any of the previous gain stages, you think this is worth the trouble to re-route the signal and fuss with the circuit?

Four ECL86 cost $240! Yeow!!! I would not spend that much on this project.

But you have two identical amplifiers on that chassis. Maybe there are two good ECL86s that you could use to fix one channel. Give this a try before giving up. I suggest fixing one channel as is first. Yes, running a guitar through the aux input will give a weak toneless sound. In order to convert to a decent low power guitar amp you need to remove V1, V2, and V3 circuitry. Basically everything before the volume control. Also remove the loudness cap/resistor from the volume control. This leaves a power amplifier. Now just build a simple two triode guitar preamp and connect the output to the volume control. Look at my PeeWee amp for a very similar conversion. You would need to do something similar. I chose a Fender preamp but a Marshall style 18 Watt Lite IIB would be another good choice for a preamp. Ampeg also has a two triode (12AX7) preamp that would be easy to add.

All this depends on getting the power amp(s) fixed. And keep in mind that it may take more than just good tubes to get the basic Truetone working. The PT may also be bad!

[Phil_S]

Ange,
I'll put in my 2¢ here. Of course, you will do as you like!

Here's how I see it. I should mention, I am a real sucker for donor amp projects and this one looks really juicy to me. This amp is a low value item even if restored, assuming all the transformers are good. Due to the cost of tubes, there is no compelling reason to restore it and if you did restore it, it's going to be a hifi, not a guitar amp. So, the first question is, do you have an end result in mind or is this something you acquired to tinker with? I'm thinking it is a tinker toy, mostly because of your attempt to cobble together an attempt to run a guitar through it. On that basis, if it were mine, I'd strip it of all the small parts and put a guitar amp in that chassis. (Save whatever you might want to reuse.)

There is plenty of room for a turret board if you like, and an extra output transformer that can be removed as a donor for yet another amp. As Steve suggests, you've got choices of known good guitar amp circuits that can easily be built here. You've got 7 noval sockets, which is more than you are going to need. I don't think I'd want to drill any of them for octal size as they are already fairly close together and placement may be less than optimal to begin with. Narrowing the space between tubes is probable not the best idea. You can, however, remove the can caps and convert those holes to octal size if the placement works. (I was wishing for picture of the top side of the chassis.) The one thing I might do for power tubes is swap sockets for a couple of 7-pin to allow use of 6AQ5's. The 6AQ5/6005 is exactly the same as a 6V6 but in a small bottle. I don't know if you can get new ones, but old ones can be had for very little money and they are fairly plentiful. If you decide you'd like to go this route, I can send you some good used or ANOS. I have a big stash of 6AQ5's that I tested and matched on my homemade tester (unless the test results fell off due to rubber band dry rot) that I'm never going to use. PM me if interested.

I'll second Steve's suggestions for any Fender, Marshall, Ampeg, or whatever preamp section you'd like to have. And, FWIW, I like Steve's idea of how to get it going as a guitar amp before you do anything else. If it really sound's just meh, maybe you don't bother, sell it on eBay, and move on to the next thing.

Good luck.

Phil

[angelodp]

Gentlemen, tinkerer, guilty as charged.... but I am hoping to get something out of this amp.

I did a check out the transformers and they seem fine. I do get the prescribed B+, but the V1 heater bias is at 8.2VDC... tad low.

I have removed the 'loudness network' .02/22k - with a mild increase in volume. I have also removed the 100k/470pf at V2 A&B (although my connection does not see that- right).

Ok so V1 and V2 are not doing me anything, so next step would be to strip out that circuit. I think the single amp makes sense and so ultimately loosing the stereo arrangement is on the agenda. I am also interested in the make over Phil-S suggests.

Steve, I will follow your suggestions and listen and then maybe move to Phil_S Idea.