Viable conversion project or not?? 1959 tape recorder with a single 6CM6 output tube

[Usable Thought]

I have one question - whether a 1959 tube tape recorder with just one 6CM6 output tube is a valid candidate for conversion to a guitar amp?

I'm fairly new to tinkering with tube guitar amps but do have some experience gained with troubleshooting noise in a Laney Cub 10 push-pull. It took several months of dead ends before I found the culprit, so I have learned how to use a scope & other basic tools; I know the safety routines to be followed; etc. Anyway I got very excited the other day at a local used goods store when I saw an old tape recorder on offer for $10 . . . with TUBES. I brought it home & opened it up just enough to see its condition. It's a Trav-Ler TT591. Rectifier is a 6X4, input is a 12AX7, PI (I'm assuming) is a 12AT7, and output is a single lonely 6CM6. There is also a 6S4A but looking at the schematic it looks like that was used only for erasing the tape. The schematic is another nice aspect - thanks to Sams, I was able to buy a complete manual & schematic for another $15. The manual is incredibly comprehensive and will be a big help in separating those circuits I want versus those I don't.

But I am wondering about this question of just one output tube. My only written guide so far to conversions is a nicely written page over on Geofex.com - Converting Integrated/PA Tube Amp into Guitar Amps, by Joe Jasniewski. At one one early on he says this about picking a good candidate for a conversion:

When looking for a suitable unit, a judicious application of the "silk purse from a sow's ear" theory is beneficial. In other words, the better the amp you start out with, in terms of size, quality and condition, the better your project will turn out in the end. I've made some real screamers out of a simple monaural 15 watt Heathkit amp, but...practically speaking, that little amp just would not cut it playing in a band. Look for at least a pair of 6L6 or 6CA7 types for output tubes; possibly a quad of 6BQ5s in an amplifier before you buy. If you come up with an integrated or PA amp that has a quad of 6L6's in the output stage, you'll be able to create quite a machine!

I've looked up the 6CM6 and from what I read, although it is a miniature, it can perform similarly within its ratings to a 6V6GTA. And I should add that my own guitar playing does NOT require loud volume nor ultra high gain etc.; I play mostly jazz with a fairly clean tone and moderate living room volumes or quieter. So a small amp would not deter me. But I thought I should ask anyway! In looking through this section of the forum, I see where Riffraff has recently fixed up a very nice looking '51 Stromberg Carlson converted PA, which also has only one output tube - but that tube is a much more powerful 6L6. So I am still left wondering: could I make a nice single-ended amp out of this Trav-Ler TT591, or is the 6CM6 not quite enough oomph, even for something like a bedroom amp?

The guts look not so bad for something this old. One question is how the PT has fared. Also, where some amps don't seem to require too much chassis modification, I am wondering whether in this case I might have to move some things around - but I really haven't thought that far yet.

Perspectives on this question are welcome - as are any suggestions for where else I might look in addition to the Geofex page for hints & tips on conversions in general, especially single-ended. I can probably cobble a lot of information together over time on my own, but as a novice and avid learner, I like reading up ahead of time. So the more reading I can my hands on, the better.

And now here are some pics -

As purchased -




Inputs at bottom right -




The guts, not too bad shape -




Schematic - looks like the output tube has 250VDC on its plate -

[Enzo]

Valid candidate? Anything that drives a speaker is a valid candidate. The venerable Fender Champ is a 12AX7 and a power tube. I don't know the 6CM6, but it is a pentode power tube driving a speaker.

You plan to put the chassis in something else? Or leave it in the tape deck? What speaker is in there, a 5x8 oval or something? 4x6?


The 6S4 is indeed the bias oscillator - says so right on the schematic. It makes the high frequency bias for the erase and record heads. I'd just pull that tube out and that eliminates any interference from the bias oscillator.

This amp is single ended, meaning one power tube, not push pull. There is no phase inverter in a single ended amp. There is no second power tube to need the inversion for. The two dual triode tubes are simply four gain stages in a row.

Like most tape recorders, there is a many-sectioned R/P switch - record/playback. The same amplifier stages are used for both purposes, but they need various changes when they are in one mode or another. The MIC input is nly live in RECORD mode, otherwise the playback head is connected to the input. All thos SPDT switches throughout the thing are parts of that R/P switch. You can make sure it is anchored in the REC position or remove it and solder jumpers in all the right places.

There are a lot of places along that signal path to massage the tone, and in fact they already did that because the needs of the recording process and playback processes are different. In fact in a couple places you may find it better to wire the RR/P in the "wrong" position at one switch or another.

[Usable Thought]

Thanks for the clarifications. Since the only tube amp I've owned (the Cub) is a push-pull, I have some learning to do about single-ended, e.g. no PI. Obvious, but it's the obvious things that often elude me.

I don't think the chassis can possibly stay in the tape deck, so it will have to move into something more suitable, likely that I will need to build. There are two 4" round speakers which I won't keep.

The switch is interesting - I have already been thinking I will want to get to know it quite well. Jumpers sound better but we'll see.

There are a lot of places along that signal path to massage the tone, and in fact they already did that because the needs of the recording process and playback processes are different. In fact in a couple places you may find it better to wire the RR/P in the "wrong" position at one switch or another.

Can you expand on this thought a little?

[olddawg]

You might be able to use it as is if you want clean and a little dirt. With most tape recorders you can monitor either the playback or the input signal. You you would just put it in input monitor mode and play. There used to be lots of uses for tube tape decks. Slap delay or to overdrive another amp. In this case, for what you say you want to do, you might want to just remove the PT and OT and start over. Find a chassis and build something Champ like. New ceramic tube sockets are cheap. You plan on using a different speaker and cabinet anyway.

[g1]

If you don't need more than living room levels, the single ended 6CM6 should be enough volume.
The remark in the article you quoted, about needing at least a pair of 6L6 or 6CA7, was for a live band in venue situations.

[Usable Thought]

You might be able to use it as is if you want clean and a little dirt. With most tape recorders you can monitor either the playback or the input signal. You you would just put it in input monitor mode and play.

Thanks, that's helpful to know.

In this case, for what you say you want to do, you might want to just remove the PT and OT and start over. Find a chassis and build something Champ like.

I'd rather keep as much as I can. If the PT is dead I'll replace, ditto OT - but the charm, I think, is in modifying a circuit but keeping its skeleton as much as possible.

[trobbins]

You may be able to find an old 12" speaker commonly used in old guitar amps in the 1960's - they were often low power and should make quite a reasonable dBA level.

You should be able to disconnect the 6CM6 anode feedback, as there may be sufficient hf roll-off in the preamps, and the bias oscillator is now gone, and there is no other output feedback.

There is certainly enough gain stages to overdrive preamps and output stage, so some preamp grid stoppers may be in order. One of the preamp stages could easily be made in to a switched 'boost' stage, for lead guitar work.

The 6X4 may be a weak link at some stage in the future for cathode-heater breakdown - the humdinger pot may save the day, although a PT CT fuse may be a good addition (as well as a mains side fuse!). If you work through the rectifier design with PSUD2, and the 6X4 current limits and the measured PT resistances, you may be able to modify the filter cap value.

The 6S4 stage could be modified to an LFO for tremolo.

[dmartn149]

I'm no expert, but I just finished a similar project. Mine used a 6aq5 power tube which is also much like a 6v6. It sounds great but not much clean volume. here is a link to the thread and a schematic in post #15. http://music-electronics-forum.com/t40347/the schem has a SS rectifier where mine has a 6x4 like yours. Hope it helps.

[Usable Thought]

Thanks! I had already bookmarked your project. Glad it went well.

[olddawg]

I'm no expert, but I just finished a similar project. Mine used a 6aq5 power tube which is also much like a 6v6. It sounds great but not much clean volume. here is a link to the thread and a schematic in post #15. http://music-electronics-forum.com/t40347/the schem has a SS rectifier where mine has a 6x4 like yours. Hope it helps.

Why not just use a SS rectifier? Would raise the B+ a bit. Might be desirable.

[dmartn149]

I just used what was already there, and I figured the B+ would already be higher than when new because of today's higher voltage at the wall.

[Enzo]

Higher mains does raise voltages, but the change from a tube rectifier to a solid state one can have even larger effect.

[Usable Thought]

Higher mains does raise voltages, but the change from a tube rectifier to a solid state one can have even larger effect.

What are the negatives, if any, of ditching the tube diode/rectifier? I know I've read something about this, but I can't remember where. Had something to do with switching - uh, a tube doesn't have as many harsh switching transients or something? But with fast diodes maybe that advantage is no longer as significant (if I even have it right).

[Justin Thomas]

I thought the main disadvantage of diode rectifier was jacking your voltages even higher, so that your filter caps can no longer hack it? IE, your B+ is now 520, but your caps are rated 450. Another possibility is that your tube's plate voltages may go above spec, but it's been somewhat quasi-scientifically established that this isn't a big a deal as long as current is kept under control... see any Deluxe Reverb/Champ from the late 70s, WELL over the 350V limit.

The only time I ever had switching noise was in my second build, where I made it fixed bias. The only extra turrets I had were right by the input, and I like to use as little shielded wire as possible. BUZZZZZZZZZZZZZZZZ...

Justin

[Enzo]

In an existing amp, replacing a tube rectifier with diodes will raise the voltage in the B+ circuit. This is a problem in some amps, and not a problem in others. As Justin points out, check your filter caps voltage rating and see if they can handle an extra 20-50v. if you were planning on replacing the filter caps anyway, then get caps of appropriate rating.

In my view, people worry too much about the tube specs in the RCA book or similar places. fender made Deluxes and other amps running tubes a full 100v over the supposed maximum, and they do not have a reputation as tube killers. So I don't worry about tubes.

Operationally, the equivalent resistance of a tube rectifier adds sag to the circuit. Sag is where the B+ drops when you play something loud like a big KWANGGG. The voltage immediately drops some amount then recovers over a brief period. the net effect is some compression, it adds some sustain. The resistance of the power transformer does this already to some extent, but the tube rectifier adds a lot more. Silicon diodes do not add any sag. Well, less than a volt of it. The transformer sag will of course remain.

Not everyone wants sag. Blues cats may love it, and metal guys not so much. That is up to you.