Precision Electronics Amp w/ 6GT5's, Convert to 6L6's?

[tablebeast]

I just bought a serious power amp that unfortunately utilises 4x 6GT5 tube in the power section. I want to know the feasability of converting it to a more common Octal base for use with a set of 6L6's or EL34's I plan on using it with Bass and Guitar mostly and possibly as a Tube PA. If conversion is not going to work, what are the chances of getting a matched quad of 6GT5's? Are there substitute tubes that would work with the Novar base? Are there adapters to use octal base tubes? Thanks in advance.

Jesse Mesa-Savage

 

[SY]

This is a sweep tube, not too similar to audio pentodes like 6L6. My guess is that a bit of searching will turn some up, probably pretty cheap, though you'll likely have to overbuy and match some yourself.

Similar tubes would include 6JT6, 6JN6, 6JR6, 6GJ5, 6HB5, though all these have different pinouts.

The nice thing about sweep tubes (besides their generally low prices at surplus) is their ruggedness.

 

[PRR]

> unfortunately utilises 4x 6GT5 tube

NOS 6GT5 is readily available at about $16 each. 6L6 cost $15 to $40 each. I say stick with 6GT5.

> what are the chances of getting a matched quad of 6GT5's?

About zero. BUT why do you need matched sets? 6GT5 was never available matched, so the amp must be designed to take about any motley set of 6GT5 that wanders in. Also a quad is less critical of balance than a pair. If you must obsess, test each tube alone and sort them into push and pull pairs so the total amp is balanced.

Can you find schematic or determine voltages and impedances? The general question "put 6L6 in place of any 6GT5 use" can't be answered "yes", but maybe the specific operating point can be handled by another tube.

> Are there substitute tubes that would work with the Novar base?

None of the Novars propspered outside TV set duty. You will not find them in matched sets or in new production. (If there is an exception, someone will hop on me.)

 

[tablebeast]

Cool, thanks. I don't know anything about these tubes, but I'm curious what they sound like. What kind of poiwer output can I expect from the quartet. I will keep the 6GT5's for weirdness factor for sure.

 

[SY]

Without more circuit data, it's tough to predict power, but a reasonable WAG would be 50 watts or so.

 

[Tom Bavis]

In the past, it was sometimes suggested to use 6GT5 as a sub for the 7868, with reduced screen voltage. Now that EH is producing the 7868, you might consider the reverse swap... requires much lower bias voltage, but the pinout is the same. And matched sets are available. The pins on the EH 7868s are slightly oversize, though, so 6GT5s probably won't fit afterwards - but then you didn't want to use them anyway?

But really, 6GT5s aren't hard to find. You would need to add separate bias adjustments, as you WON'T find matched ones easily.

 

[nuvistordave]

...me and my SOFIA curve tracer love Sweep Tubes. But if you lack that simply put a small cathode resistor and match tubes that have the same bias current. That will at least balance the output transformer current; and the transconductance tends to follow the cathode current so that is much better than nothing.

 

[smoking-amp]

I see 6GT5 tubes available for $15 at ESRC and Vacuumtubes.net.

These tubes are quite similar (except for base) to the 6JN6 tube (also 6GV5 and 12/6GE5) that was originally used in the DCPP amplifier and well liked.

Considering that you probably have the -only- amplifier that uses 6GT5 tubes, they should be around a while.

I you want to put some "monster" pin compatible tubes in the amplifier, try 36/6MC6. They will need over twice the heater power and a plate cap wired to the old plate, pin 9 (best to isolate from the old pin 9 on the socket with an insulated standoff).
Not that it will increase the power any, without other changes, like the power supply and OT. It would replace two 6GT5 tubes in parallel, if that is what the amplifier was using, avoiding any matching except to the other P-P side.

Well, it looks like it does use paralleled 6GT5 tubes:
http://www.angelfire.com/vt/audio/g101a_f106c.gif
Estimate a 3000 Ohm P to P OT for 100 Watts out with 440V B+

 

[smoking-amp]

Considering resistive losses in the OT, and that they used 160V for Vg2 (rather than the datasheet 150V curves), then the OT primary Z is probably more like 2700 Ohms P to P.

Looks like the 6GT5 tubes were pushed fairly hard to make the 100 Watt amplifier rating. There are some suggestions online about reducing the idle current (via increased bias -Vg1) to conserve tube life.