Tube guilt

[7N7]

Originally posted by Geek


*currently sipping some BC VQA white*

A toast to you lot!


Hi Miles,



Not to mention some guitar DIY'ers have rediscovered this beauty. The boutique market is picking them up as an alternative to outrageously priced 6L6's.

Cheers!

Hello.

Always pleased to find out about "new" types, but is 6BQ6GTB suitable as a replacement for 6L6? The specification (GE) states Vg2
max 200V and Pa max 11W, both a long way from even a 6L6GA never mind the guitarists' favourites such as 6L6GC, 7581A etc.

The problem with so many line output types is the low screen voltage limit. There is a clue in the GE specification where it is stated in the introduction "...high ratio of plate to screen current." I presume that one reason for the trend to low screen maximum voltages in these these later types was that as techniques improved it was possible to reduce manufacturing costs in TV.

7N7

[Miles Prower]

Originally posted by 7N7


Hello.

Always pleased to find out about "new" types, but is 6BQ6GTB suitable as a replacement for 6L6?

As a drop-in replacement: no way Jose; not a chance Sundance. The 6BQ6 likes to operate at higher currents, and lower Rl's than a 6L6. As a design replacement, then yes, it does substitute for a 6L6 in terms of output power (37W without spec busting) and sonic performance, which is quite good even before you add the gNFB.

The specification (GE) states Vg2
max 200V and Pa max 11W, both a long way from even a 6L6GA never mind the guitarists' favourites such as 6L6GC, 7581A etc.

That Pa rating is very conservative. I've run these at twice the rated Pd without red-plating them. There was a design for a Portugese ham rig that used these at some 700Vdc to produce some 70W of audio for an AM modulator. Being that this was intended for horizontal deflection duty, they're tough enough to stand whatever abuse an axeman can inflict on them.

The problem with so many line output types is the low screen voltage limit. There is a clue in the GE specification where it is stated in the introduction "...high ratio of plate to screen current." I presume that one reason for the trend to low screen maximum voltages in these these later types was that as techniques improved it was possible to reduce manufacturing costs in TV.

Not a prob, but this means that the type isn't suitable for Ultralinear operation. I don't use that, and believe it's one of those audiophool trends for which overblown claims are made. Stick with screen regulation, and you're good to go.

[7N7]

Thanks Miles,

More interesting information. I don't agree though, about ultra-linear, in the traditional sense because I like the idea of the screen grids being integrated as it were, into the dynamic of the amplifier. Fascinating how so many valves can be run way over their ratings (NOT E55L though - I know...)

I once built an amplifier that had DC coupled pentode cathode followers driving the output stage (6528a p-p). The screens were linked to the cathodes via an emitter follower. It worked well. Another prototype used 7N7 with E810F (7788) pentode cathode followers configured as above. It was to drive triode strapped dual beam tetrodes (YL1071/8116). Sadly I never finished the amplifier, but the driver stage was very effective: large voltage swing and low distortion.

7N7

[SY]

but is 6BQ6GTB suitable as a replacement for 6L6?

It depends on the 6BQ6. The ones I'm currently using (the "super" 6BG6 from SND) have internals which are seemingly identical to the NOS 7027s I have on hand. I've run them at 30W idling with no issues and not a hint of plate color. Tubelab has shamefully abused them and they didn't complain.

[7N7]

Originally posted by SY


It depends on the 6BQ6. The ones I'm currently using (the "super" 6BG6 from SND) have internals which are seemingly identical to the NOS 7027s I have on hand. I've run them at 30W idling with no issues and not a hint of plate color. Tubelab has shamefully abused them and they didn't complain.

Thanks.

6BG6 though is rated differently from 6BQ6: it is a 20W type with a screen limit of 350V.

I had heard of SND's special ones and indeed read TubeLab's always entertaining pieces about serious abuse!

7N7

[SY]

My fault, I need to read better.

[rcavictim]

Originally posted by fauxpas
Hey! The sun is finite and I don't feel at all guilty getting burnt by it...

Yeah but does anyone feel guilty about leaving it burn all night?

Also you have to wonder if the way our government moves the sun back and forth 15 degrees in the sky twice each year has any detrimental effect on the lifetime of this big source of all energy and life here. I`d support a class action lawsuit against the government for unknown damage caused the sun by totally unnecessary and confusion generating Daylight Savings/ Daylight Wasting Time changes.

[Miles Prower]

Originally posted by 7N7
I don't agree though, about ultra-linear, in the traditional sense because I like the idea of the screen grids being integrated as it were, into the dynamic of the amplifier.

Ultralinear is really nothing more than a form of local feedback. It accomplishes the exact same thing as providing feedback from the plates to the grids. No more, no less: it doesn't live up to a lot of claims made for it. The only difference is that plate-to-grid local feedabck is useable with those types (807, 6L6, most horizontal deflection types and RF types) that require significantly lower screen voltages. Furthermore, you can adjust the level of feedback as needed for whatever type you're using. With ultralinear, you are at the mercy of the OPT manufacturer and can only hope they got it right. Poorly made ultralinear OPTs are a cause of major headaches with instability, xfmr ringing, and poor sonics.

I definitely prefer plate-to-grid local feedback.

[7N7]

Yes Miles, I take your point about the fact that local feedback enables the use of 807 etc.

As I tried to imply in my last message, I think that relating what happens at the cathode to the screen is an area that might be profitably explored. This was done in the Quad II via the output transformer and I believe that some Macintosh models used a similar idea.

7N7

[TubeHead Johnny]

One of the greatest things about DIYing is, you can aways change it to work with a tube that's less finite.