Why run 6L6's so cool in a Fender 30?

[Randall]

I have a Fender 30 on my bench. It is a 6L6 amp with lots of pull pots for channel switching, boost, etc. It's biased at 22mA @ 417v on the plates. The schematic shows -46v for bias. Cranked all the way, I can only get 39 -40mA at idle. Anyone have an opinion on if/why Fender wants this amp to run so cool?

http://www.webphix.com/schematic%20h...r_30_schem.pdf

[Enzo]

And why not? This whole 70% thing is a latter day rule of thumb, not something from the tube makers or from Fender. The amp was not intended for the power tubes to break up. Tone was in the preamp, the power amp was clean and strong.

[Randall]

Thank you Enzo, I suspected this may be the case, but I have no experience with this amp. It's great to have such experienced and helpful guys like you around.

[pdf64]

Would you mind checking what the OT primary impedance is?
They may be limiting the power via the OT primary being higher than normal (eg 6k6) / the lowish 417 VB+ / sag from the 5U4.
The 5U4 may have a hard time with the 100uF of reservoir cap and the standby switch; it may be beneficial for tube life to re-arrange the switch so that it's after the reservoir.
Pete

[Randall]

I have 147 ohms on the OT primary. The two super reverbs I have opened up have 98 and 104 ohms, and a ClassicTone 6L6 OT on the shelf has 78 ohms.

[pdf64]

Sorry I was thinking of the actual ac primary impedance, eg 4k , 6k6
To work that out, feed a signal into one side and see what you get out of the other - that gives the voltage ratio; eg feed 6.3V across the blue and brown primary, get 0.22V across the secondary black to green, ratio = 28.7).
The square of that is the impedance ratio, then mulitply that by a known design impedance (eg 8 ohm secondary) to get the unknown value, eg 8 x 28.7 = 6k6 primary.
Or feed 1Vac from a sig gen into the secondary and measure what primary measures, to get the ratio that way.
Thanks - Pete

[Randall]

I get 4K. 6.9 vac into primary yields 0.32 vac on secondary for a voltage ratio of 22.26. 22.26 squared = impedance ratio of 495.5. 495.5 x 8 = 3,964. If I have this right, I believe there is an error in your above post stating "8 x 28.7 = 6k6 primary".

[pdf64]

Ha, apologies, well spotted, it should have been
8 x 28.7 x 28.7
Many thanks for that, it's a fairly unusual transformer set on that amp so your test has cleared that aspect of it up.
I know that when I first read how to test for this, I went and checked all the ones I could get at.
So if you're similarly inspired, have fun!
And report back any unusual results.

Anyway, my guess is that it's the 5U4 sagging under load which is that main cause of it only being 30 watts.
My Fender type amp (420VB+ 4k2 p-p 6L6) with a GZ34 rectifier manages nearly 40 watts.
What does its VB+ read when the amp is running at max clean output?
Pete

[Gingertube]

It has to be a "sound" thing.
The Rg1 resistor values of 220K say that it would be OK to bias the amp right up to that 70% mark (21 Watts per tube). Rg1 would need to reduce to 100K to bias any hotter than that.

The bias point sets where the amp makes transistion from Class A (both tubes operating) to Class AB (where each tube cuts off for part of the cycle). When one tube cuts off the output impedance of the amplifier goes to 4 times the value it has when in Class A. That means that above a certain signal level the amp suddenly does not damp its speakers as much (on the signal peaks). Good players swear that they can pick this Class A to Class AB transition and this is one arguement why some players like a smaller power amp so that they can push the amp to this point more easily.

This low bias low power option on the power amp is designed to get to that transition very easily without ear bleeds.

It may be "one of those tales" but it is widely reported that Angus from AC/DC prefers a 50W Amp over a 100W for this reason.

Cheers,
Ian