Bias vary trem & Fixed bias - what optimum combination?

[Merlinb]

Maybe 1k5 grid stoppers on the output tubes would help too.

Woah woah woah! I didn't realise you'd copied the 5F6 like-for-like! That's one of Fender's design blunders- you absolutely need grid stoppers there, or you WILL get blocking! Personally I would up them to 4.7k minimum. I would be very surprised if that isn't where your problem is coming from, those 100nF coupling caps are HUGE!

[MWJB]

+1 on the grid stoppers...but 4.7K might be overkill (depends on how good your layout is, it is possible to "get away" with none, Fender didn't install them as a matter of course on fixed bias, LTPI, 2x6L6 Fenders until the BF era), use the smallest value that gives you proper performance. I'd start at 1.5-2.2K.

[Merlinb]

use the smallest value that gives you proper performance. I'd start at 1.5-2.2K.

I don't wish to start a flame war, but that's a bit like saying "use the smallest brake discs that will stop the car".
You shouldn't be using the minimum value (there no logic in it), you should be using a "healthy value". And before you worry, Tubeswell, they have to be pretty enormous before they will have any audible effect (except the reduction of blocking of course! ).

[MWJB]

"I don't wish to start a flame war", & I don't wish to propagate one, but...

A car that doesn't stop is a car that you cannot drive. It is dangerous. An amp that sounds fine & doesn't suffer blocking with a smaller grid stopper is an amp that works & is probaby laid out well...in short, what the amp designer should be trying to do in the first place. So it's NOT the same, it's more like using lighter engine materials/reducing unsprung weight.

What are the size of the grid stopper on a 59 RI Bassman? (47R just to save you time looking it up).

Do I fit them? Sure I do, usually 1.5K, if I run out I use 2.2K...if you typically need bigger than that in a 6L6 Fender style circuit it's because you have done something wrong/new/different somewhere else.

[Merlinb]

An amp that sounds fine & doesn't suffer blocking with a smaller grid stopper is an amp that works & is probaby laid out well...

But the problem is that layout has very little to do with grid stoppers on the power valves. Whether the amp suffers problems there is mainly dependant on the gm and interelectrode capactiance of the power valves, and on the primary and leakage inductance and capacitance of the OT, over which the deigner has no control. (And we probably aren't using the same OT Fender was using back then...). Just because the "seems" to work doesn't mean it isn't oscillating at HF (remember, HF oscillation can be inaudible, but will still burn out voice coils). And it doesn't mean that it will stay stable as the valves age or when you plug in a different set of valves with slightly different characteristics.
Using the smallest grid stopper you can 'get away with' is like using the smallest brakes on the car that will "just do the job". It may be fine now, but long term reliability is questionable. You should always use values which are large enough to protect against natural component (mostly valve) tollerances.

if you typically need bigger than that in a 6L6 Fender style circuit it's because you have done something wrong/new/different somewhere else.

That simply isn't true. Many power valves need values as high as 10k to make them stable with a particular output transformer (particularly when new = high gm) and no two output transformers are alike either. If you find you need bigger than Fender used it's not because you've done something wrong, its because you're not using the exact same valves or OT that Fender used. (Marshall discovered this pretty quickly!)

I also don't see any logical reason for using the smallest possible value. If zero is not enough, 1k does the job, and 2k also does the job and doesn't sound any different, then why would you want to always use the 1k "borderline" option?

[MWJB]

I think that there are enough, well used, 40-50yr old amps, with the original OTs & no grid stoppers, to suggest that there's more going on than simply perameters in the power tubes, over which the designers "have no control".

We're not talking about "many power valves", we're talking about 6L6 style tubes in a largely proven topology...one in which I have never seen grid stoppers over 1.5K. Indeed, I own a EL84 amp with a 27K grid stopper & I see no reason to change that.

Agreed difference between 1K & 2K may be moot, but if you have to use nearer 10K you are likely just masking symptoms, not dealing with the cause. Likewise if a different brand of 6L6/OT (assuming that it's of a suitable rating & close enough primary-Z) causes you trouble.

[Merlinb]

True, beam tetrodes have lower gm than similar pentodes and therefore can use smaller values. But I would argue that plenty of those 50-60 year old amps are not stable, never were, and shouldn't be copied because of it. They are not all 'proven' designs, they are just bad amps that seem to work, either by dumb luck, or because their symptoms were masked by something else (such as less sensitive speakers than we have today, and by all that background hum and hiss).
There are innumerate vintage amps which just sound terrible today (because they weren't designed to be overdriven), and desighers then were just as liable to overlook something (or cut corners) as they are today. JUst because an amp is old, or came from the sacred house of Fender, does not mean it is well designed or beyond improvement.

Grid stoppers are not a band-aid; they do not just cover symptoms, they are the solution. Unfortunately many enthusiasts don't recognise them for what they are and what/how they do it. (See neutralisation in any Terman textbook). When you copy an old amp using a new OT and new components and trouble arises, it's not because there's something wrong with those components, it because the old design wasn't designed to cope with a sufficiently wide spread of component tollerances. It was designed to use that OT with those speakers and those high ESR PSU caps, etc. Modern electronics has changed all that. Today we (should) design things that are univerally reliable.

[MWJB]

We're getting of topic here...& I should know better but...

The tweed bassman IS a proven design, yes, Fenders have their shortcomings & yes there is sometimes room for improvement. You might also want to consider skill & experience as well as "dumb luck". What background hum, & hiss? Many vintage Fenders hum & hiss less than most modern amps.

The modern, hi quality, repros don't seem to need excessively large grid stoppers either.

This is becoming a discussion about a discussion, thus this will be my last post on this matter.

[tubeswell]

Thank you MWJB and Merlin for this information. 1k5 (right on the grid pins) seems to have removed the audible 'blocking' distortion (which was only very slight on some of the bass notes anyway) although I have not 'dimed' the amp yet. I will experiment with higher values to see if I can detect changes in the sound.

FWIW it is quite a 'bright' sounding amp (at the moment it has a Telefunken ECC83 in V1) altho' it seems to have plenty of bass response as well. FWIW the OT (custom made) is fairly robust and was ordered as 50W, but in reality can probably handle quite a bit more than that.

[TheTinMan]

I'm curious having just finished an AB763 Super Reverb with 6G16 bias vary tremolo using Hoffman's approach.

There are two "extra" components in your tremolo circuit. There's a .02uf cap connecting the plate & the cathode on the first triode. Someone just recommended that change to me but couldn't explain what it does other than "improve the sound". The second change is the 470k resistor connecting the grid and the plate of the buffer triode. What is its purpose?

There's nothing new in the tremolo circuit I used. It's straight off Hoffman's diagrams with the tweaks Tubenit linked to (i.e. reduce 470K off buffer cathode to 220K and change .1uf to .047).

Don't know if this helps,

Chip

[Merlinb]

There's a .02uf cap connecting the plate & the cathode on the first triode. Someone just recommended that change to me but couldn't explain what it does other than "improve the sound".

It filters out harmonics to make a 'purer' sine wave. You can read about it at Aiken: Designing Phase Shift Oscillators
I don't think it makes any useful difference, personally.

The second change is the 470k resistor connecting the grid and the plate of the buffer triode. What is its purpose?

That's just the load resistor for the oscillator! It's just drawn in a funny way.

[tubeswell]

I ended up taking that substituting that .02uF cap for a .01uF, then a 500pF, and then I took it out completely. I also reduced the 470k series resistor (from the CF cathode coupling cap to the depth pot) to 220k. (So the LFO circuit is now closer to a 6G9-B than a 6G16). Man what a difference! Now the trem has a great range of depth, even when the output tubes are biased hotter.