Mullard 5-20 with 6L6GC's?

[thevalvekid]

Hi all,
I'm starting my first scratch-built tube amps, and after hearing one in the flesh, I've settled on a 5-20 as a base. Wow, what a lovely sound.

I'm a little stuck on optimizing the circuit for 6L6GC's (my preferred power tube), is there anything it would need to work best for these tubes?

I'm also looking at picking up some iron for them, namely a pair of 6.6k OPT's, probably Hammond 1650G's, and matching Hammond 374BX PT's.
Would these be suitable for the 5-20?

Any suggestions?

Thanks!

[SY]

It's a good tube choice. With a 6k6 load, 450V, and fixed bias, you can idle at 50mA and get decent tube life with low distortion. Expect 40W in ultralinear, a bit more in pentode.

The 5-20 can be improved a bit by using matched plate resistors on the phase splitter and a CCS in the cathode. But this is shaving things pretty fine- it's a good amp as-is.

[DF96]

Is there any truth in the idea that beam tetrodes, like 6L6, have a lower input impedance than pentodes (e.g. EL34) so can't be driven from high impedance valves like ECC83/12AX7? I have seen this stated in various places, but never managed to find any hard data or plausible explanation. Is it just a myth propagated by all you (present company excepted) US ECC83/pentode haters?

[sreten]

Lundahl Transformers - 30W PP amplifier designed by Claus Byrith

Hi, definitely IMO worth a read regarding the 520 (and 510), rgds, sreten.

[kevinkr]

Originally Posted by DF96

Is there any truth in the idea that beam tetrodes, like 6L6, have a lower input impedance than pentodes (e.g. EL34) so can't be driven from high impedance valves like ECC83/12AX7? I have seen this stated in various places, but never managed to find any hard data or plausible explanation. Is it just a myth propagated by all you (present company excepted) US ECC83/pentode haters?

This is the first time I have ever heard this, so I am not sure who is propagating this idea. I've not found evidence to support it in any short search either or from direct experience.. Would not the KT-66 and 77 be the subject of a similar conjecture? (Kinkless Tetrode?) I suspect a myth.. I've seen plenty of 5-20 with KT-66, 5881, and 7581..

I like ECC83 and certain pentodes a lot... Mainly as error amplifiers in my voltage regulators.. Actually used properly the ECC83 is a very linear device, just needs a little help driving a load - not always an issue depending on circuit design..

[Eli Duttman]

The Mullard topology is excellent and will yield good results with O/P trafos of modest cost, like Hammond. However, given its high R_P and low gm, the 12AX7/ECC83 is a POOR choice for long tailed pair (LTP) phase splitter duty. A 12AT7/ECC81 as the LTP will yield approx. = stage gain, while exhibiting low R_P and high gm.

SY's advice about a LTP tail CCS is spot on. A 10M45S integrated circuit is quite adequate for the job, but a more sophisticated current sink is, of course, OK.

[DF96]

I think I may have seen the myth on Audio Asylum. Maybe I should take less notice of what I see there. I just wondered whether there was smoke without fire. Glad to be reassured.

My understanding is that ECC83 is a good choice for LTP phase splitter as it has high and stable mu. It is mu which primarily determines the ratio of differential-mode gain to common-mode gain, which is the main issue with an LTP phase splitter. In fact the mu is high enough that a tail CCS is not strictly necessary, whereas it is necessary with almost any lower gain valve. The snag with ECC83 is that it needs to be run rather too close to the grid current region so imbalance can result.

[toprepairman]

This is a bit of a myth based most likely on the effect of interelectrode capacitance within the valve.
This capacitance is made worse by the effect of Cag times the mu (voltage gain) of the valve. This is known as the Miller effect. If say you have control grid to anode capacitance of 10pF and the voltage gain is 20 then the effective capacitance from grid to anode is 200pF. This capacitance is effectively 'seen' by the driving valve and becomes a loading impedance, but the effect is only really significant at radio frequencies.
The difference between a 6L6 and EL34 is much less then the different operating modes, ie, triode mode is much worse then pentode, with ultralinear somewhere in between, depending on tapping ratios.
Regards
Henry

[SY]

The 5-20 doesn't run the output tubes as triodes. The input capacitances are really, really small. 200pF is huge and VERY significant for audio purposes.

[DF96]

No I wasn't thinking of Miller effect. I assumed whoever started the myth was talking about grid current, but I can't think of any reasonable explanation why that would be worse in a beam tetrode than a pentode. Always nice to kill a myth!