Can this circuit be guitarized and rebuild without interstage transformer ?

[kagliostro]

As I can get some 6bq6gtb

I was interested to see if they can be used as final tubes for audio
(I've read somewere that it can be done)

looking for schematic that use this tube

I've find only this (as complete and tested schematic)

can this be guitarized and rebuild without the interstage transformer ?

what about a triode before the pentode input ?

Thanks

Kagliostro

p.s.: in the pdf file there is a complete description of this modulator

[eleventeen]

Seems like an interesting project, but I am unclear...do you have this actual piece of equipment and wish to modify it, or, do you wish to make (from scratch) a more conventional guitar amp just using 6BQ6 output tubes?

You should be able to use those tubes in a conventional push-pull design fed by a phase inverter, they are probably somewhat tougher than 6L6's. They can certainly withstand higher voltages and peak currents than older 6L6/6L6G, but probably not as much as 6L6GB/GC. The REAL high-power TV horizontal output tube is a 6*D*Q6. Make sure you insulate those plate connections!

If it were me, I would get a dual-6L6 audio amp and make socket adapters to use your 6BQ6 rather than building a whole amp from scratch...unless you're ready to abandon the 6BQ6 project and switch back to conventional 6L6 power.

Plus...you're not making your job easy having only 300 VDC.

[FYL]

As I can get some 6bq6gtb

I was interested to see if they can be used as final tubes for audio

The 6BQ6GTB was a sweep tube, relatively low working screen voltage & meager plate dissipation. I'd use them just like 6V6GT's, with a tight screen supply and a 4K pri OT.

looking for schematic that use this tube

You may begin with any classic amp using 6V6GT's.

[kagliostro]

Thanks Eleventeen & FYL

the intention is to use some 6bq6gtb I got

I was looking for an usable schematic and the only one I find was tose of the modulator I posted

a friend told me I can use those tubes with 350v on plates - 150v (stabilized) on screens - 35ma current
and obtain near 37W

he also told me that I must not use 6bq6 tubes that way, he told only 6bq6gtb can used

I've also read that the tube used that way can be considered a 6v6 on steroids also that tube need less swing to be pushed

do you think that a PI designed for el84 tubes is to be used or I must think to use a PI designed for 6v6 ?

I know what is transconductance, but I'm not still able to understand well tube datasheets

other question, do you think that this tubes can be used in SE amps ?

about other tubes (6dq6) I've some el36 (and maybe also PL36, don't remember well) - for the use of those tubes I've referring schematics

Thanks

Kagliostro

[eleventeen]

If I may be forgiven for referring you to another forum, I recall seeing some chatter back and forth about using 6BQ6's/6DQ6 as output tubes in a hi-fi forum, so I searched for them.

http://www.diytube.com/phpBB2/viewtopic.php?t=3330&sid=79fa0dd777ae5b6849ffa8baa6c9bd7e

http://www.enjoythemusic.com/magazine/manufacture/1108/  << Note that this is for 6*D*Q6 which is a MUCH stronger sweep tube than a 6BQ6

I think you will find some more stuff on 6BQ6 as audio amps if you search for "6BQ6 audio" or variations on this idea.

6BQ6 would be considered an "obsolete" tube (if that isn't already redundant) but as FYL said, this tube was used as a horizontal output tube in TV sets. But remembering the many hundreds of TV sets I took apart during my misspent youth, 6BQ6 started to disappear in the early 50's and became replaced by 6*D*Q6 probably by the late 50's. Those dates are just a guess. The 6DQ6 is a substantially more powerful and durable tube.

I have no doubt that these tubes can be used for audio output tubes, and I stand corrected that a 6BQ6 is probably more like a 6V6 than a 6L6.

[PRR]

It's a low-Mu tube. That means it is good for passing HIGH current at moderate voltage. In particular, to pass 260mA with only 60V at the plate. It will also cut-off well. Good switch.

It ISN'T made to pass fairly high current at fairly high voltage, because it has a low plate dissipation rating. But that's what we want in audio: able to stand part-current part-voltage and fairly high dissipation.

Compared to 6V6: it will work about the same _IF_ you power the Screen grid with about 4/10th of what you would give a 6V6 screen. 4/10 is the ratio of the two tubes' triode Mu (sheets say like 4.3 and 9.8).

I just penciled a loadline at 300V plate 6KCT push-pull load. You need at least 120V on Screen, about -18V grid-cathode. Output is something over 20 Watts per pair.

> 350v on plates - 150v on screens - 35ma current and obtain near 37W

This appears to exceed the spec-sheet rating for 6BQ6 (even GTB).

TV H-sweep duty is tricky. If everything is right, dissipation is low; if anything goes a little wrong, dissipation soars. The tube maker must leave a large margin for circuit losses and mis-tunings. I have no doubt that this tube will handle much more heat in the plate. The 37W audio claim is probably quite safe for good (large-plate) 6BQ6.

Compare to 6V6. With 8K load you get 14 watts, similar grid drive, and the Screen voltage should be around 270V-300V, SAME as plate supply (or a bit less for filtering). And less than half the heater power. (6BQ6 is mostly a BIG cathode, and that means a big heater.)

Back to the 6BQ6. Where are you going to get 120V? The current demand will be 4mA idle and maybe 20mA at full power. At idle you could use a 45K resistor, but at full roar you need just 9K. A simple resistor dropper will not work.

You could wire the Screen as in the QST plan, with a shunt regulator which pulls as much as the maximum screen current, plus a little more, all the time. This is a steady 13 Watt waste. And a high-watt resistor working at high voltage. And they use the gas-tube, which has not been made in years.

There's no real need for _regulated_ screen voltage. But it has to be much less than reasonable plate voltage, and must not sag much for 4mA-20mA load.

You could build a separate +130V DC supply. With vacuum rectifiers, that's messy.

You could build a resistor voltage divider. For low sag, it must diddipate far more power than the screen grid needs.

You could use a modern power MOSFET to drop 300V to 130V. But that's 5 watts of heat at full roar.

You could use a 230V CT winding with a bridge, or a 115V AC winding with a doubler, to get +300V with a solid tap at +150V for screen.

You could use the lower-Mu 6V6. With screen raised to plate voltage it passes just-enough (not too-much) current for the power its plate can dissipate in audio service. Unless you are going to build a thousand amps, any cost for 6V6 is less than the design cost to adapt to 6BQ6.

> can this be guitarized and rebuild without the interstage transformer ?
> can be used in SE amps ?

You can use any tube to do any work. You may need more than one: Type '76 is a nice triode but you would need about five of them in a row to boost guitar up to power-tube drive level. 6AU6 is a fine small pentode, but you woud need dozens of them to make enough Watts to play on stage. Just because you "can" does not mean "good". Some tubes are a better fit than others.

SE 6BQ6 amp ---"the 6DJ8 tube drive stage is not enough." 

[kagliostro]

Thanks again Eleventeen for the considerations and links

Many thanks PRR for your advice and explanations

I just penciled a loadline at 300V plate 6KCT push-pull load. You need at least 120V on Screen, about -18V grid-cathode. Output is something over 20 Watts per pair.

so using a 220v / 110v + 110v transformer will result in ~308v (using the 220v connection) for B+ and ~154v (using the 110v connection) for screens that will be good for ~20W

screen stabilisation is no need ??

However I think the best way to obtain the voltage for screens is to have a 110v AC winding from transformer, instead to use resistors or mosfet to drop B+

You could build a separate +130V DC supply. With vacuum rectifiers, that's messy.

With vacuum rectifiers ...... not simply a SS bridge ?

What about the PI ? must it be as for 6v6 designs or is better to use a PI planned for el84 ?

other question what do you think about the use of the 6an8 as V1 & V2 ? I've seen that tube in some old RCF PA amps, what about the use for guitar amps ?

Thanks again

Kagliostro

[PRR]

> must it be as for 6v6 designs or is better to use a PI planned for el84 ?

What is the difference?

Oh, there's some traditional difference because 6V6 amps were designed in America and EL84 amps were designed in Europe. And because EL84 does not need as much drive as 6V6. But the differences are minor, and you can generally swap them.

> what do you think about the use of the 6an8

Speech voice microphone needs a lot of gain, and is unlikely to overload one high-gain stage. The pentode does that. Driver transformer needs a strong triode, the triode side does that. Perfect for their application. Would "work" as guitar amp. The pentode would overload on hot guitar, and the driver transformer is not common today, and not common in guitar amps because it distorts different and apparently not in a way we like.

Guitar amp is:
Preamp and tone-control stages
Driver
Output stage

You can generally mix-and-match.

The driver must be "suitable" for the power output stage. 2A3 power triodes need BIG voltage swings. Fix-bias 6550 should have low-value grid resistors and large current from driver. EL84 does not need big voltage, and a "too good" driver will over-over-load the grids, we sometimes have to "weaken" a driver for EL84. But mostly you can cut/paste one power tube into plans for another, with suitable choice of voltages and loads.

[kagliostro]

Thanks PRR

Guitar amp is:
Preamp and tone-control stages
Driver
Output stage

You can generally mix-and-match.

assuming that Driver is PI, if I want to use the Output stage of certain amp as a stand alone Power amp can I cut the schematic before the PI and retain PI and final tubes to build a Power Amp ?

Must consider to put an amplifier stage before the PI (as a general rule) or not ?

and thinking to preamp section if I cut the schematic the same way and retain the preamplifier part there is need to make mods (as a general rule) or the preamp will work fine as is ?

Thanks

Kagliostro

[RicharD]

Do you have this box?  My first question would be the output transformer.  It looks like an interstage transformer too.  Seems like you are mainly wanting to use 6BQ6 tubes.  You're money ahead using 6V6's IMO.  See how they used a VR150 to regulate the screen?  This tube can't handle a wild screen and geetar amps sound better with a wild screen.  Now the 6AN8 is a cool tube.  The pentode side has a higher transconductance than a 6AU6 but effectively the same.  The triode side is is close enough to a 12AU7.  The 5V4 is undesireable due to lowish output current and low filter capability.

[kagliostro]

Hi Butterylicious

Do you have this box?

No I haven't although I would have liked to find one

As I can get some 6bq6gtb

I was interested to see if they can be used as final tubes for audio
(I've read somewere that it can be done)

looking for schematic that use this tube

I've find only this (as complete and tested schematic)

can this be guitarized and rebuild without the interstage transformer ?

I got the tubes - I've 22 Thunsgram and 3 Fivre NOS NIB tubes (on the cheap for our market)

My interest was about to find a schematic as to make a copy & paste with other standard guitar schematics to obtain an amp that use this cheap tubes as final tubes

My first question would be the output transformer.

Yes that schematic is an AM modulator for RF tube transmitter, so the secondary of that transformer is far from the impedance we use for audio output

Someone told me that I can use as OT a (primary) 4Kohm / 4.4Kohm CT transformer (of course standard secondary) - PRR told also a 6Kohm CT OT can be used (at near 300v B+)

See how they used a VR150 to regulate the screen?

Yes that thing is something I've understand reading the schematic

This tube can't handle a wild screen and geetar amps sound better with a wild screen

That isn't a good news, does it mean that this tubes will be better used in a clean amp instead in a High gain one ?

Now the 6AN8 is a cool tube

To me it will be interesting to try a triode stage  as 12a*7 (for V1) the pentode in 6an8 (as V2) using the triode in the 6an8 (as V3) that will be the PI --  Tonestack between V1 & V2 or V2 & V3 ??

what do you think about ?

Thanks

Kagliostro