5F11 sounds great, trem works - but very low output.

[Josh_B]

I built a 5F11 using Deluxe power and output transformers from Triode Electronics. I made a negative bias supply that comes through a 220K dropping resistor from pin 4 of the rectifier and supplies a perfect -31 volts to the junction of the two 220K resistors feeding the power tubes grids.
In fact, all of the voltages in the amp ;ook goo, but about 15-20% high, compared to the original Fender layout. The tremolo is to die for, and the tone is outstanding.
The only change I made to the circuit was to use a 40uf main filter cap and a 3MRA pot for the Speed control.
BUT...
This thing only seems to be putting out one or two watts!!! It's very very quiet. I was expecting around the same output as a 5E3.
What's the deal there?

[Josh_B]

I got to thinking... having two speaker jacks (4 ohm and 8 ohm) and having the negative feedback loop hooked up to both might be messing me up. I unhooked one jack and WOOHOO! Nice sound and pretty loud, too!!

With 420 volts of B+, using 5.1k and 33k dropping resistors gave me really good voltage readings throughout the amp - almost exactly the same as the original Fender specs!

[tubeswell]

I got to thinking... having two speaker jacks (4 ohm and 8 ohm) and having the negative feedback loop hooked up to both might be messing me up. I unhooked one jack and WOOHOO! Nice sound and pretty loud, too!!

Yep you only need the one NFB off the highest value (8 Ohm) tap. This will pick up the 4 Ohm tap (because the 4 Ohm is just a tap off the 8 Ohm winding) - albeit that the NFB will have a tad more resistance from the extra 4 Ohms in that 8 Ohm winding that the NFB passes through when the 4 Ohm tap is utilised. - If you follow me.

Glad you like it BTW - it looks like a very tidy job - very nice

The other thing to check is the output tube plate dissipation. This is more critical than just assuming that the -31V will be supplying the appropriate bias for the output tubes. This is because alot of things can add up to influencing the bias level, not just the -ve voltage feed at the output tube grids.

With a pair of 6V6s fixed bias in push-pull you want to be aiming somewhere between 10W plate dissipation per tube (9.8W for a safe long-tube-life-boring tone - 11W for a sexy tone - Do I hear 12W? ;-). One rule-of-thumb way of working this out is to put a 1R 1% metal film resistor between each output tube cathode and the ground, and measure the mV across each end of the 1R resistors. Since 1V/1R = 1A, then XmV will equal XmA, so a reading of say 29mV would show that 29mA current was being drawn through that tube (this will of course include the screen current, as well as the plate current). If you then have say 335V on each plate, then 335V*.029A = 9.7W So if we roughly subtract a few (say 5?) mA for each screen, we could bump the -ve supply up a tad to get a bit more juice so that we get about 10W or 11W with this method.

A more accurate (and potentially more dangerous) way is with the amp switched off and discharged, to measure the resistance from each plate pin to the OT center tap, and then with the amp switched back on again carefully measure the voltage across these same two points, and then divide this voltage by the resistance you measured to get the plate current, and then multiply this plate current by the plate voltage-to-ground to get the plate dissipation.

The text books say you should have the tubes biased to idle at about 70% of their maximum rated dissipation for fixed bias output stages. You can go higher (to say 85%) if you've got good tubes, and it will sound better. Some 6V6s like NOS RCA , or modern "Tung-Sol re-issue" 6V6GT, or JJ 6V6s, can handle higher voltages and in reality have a maximum rating of about 14W, this means biasing these tubes to about 10W for the 'safe' rating, or 12W for the slightly-higher-risk-but-better-tone rating. I don't know how solid EH6V6s are - It might pay to assume a max rating of 12W for those (which means a bias of 8.4W(70%) -10.2W (85%)

[Josh_B]

Wow! That's a lot of great info. I never knew the formula for calculating plate dissipation using the resistance of the OT transformer. Thanks very much!

Do bias meters take that into account, or do they "ballpark" it? I have a Bias Rite that I use, but I always wondered how it figures in the internal resistance of the OT.
Maybe that's a silly question, but I'm a relative newbie.
Thanks!

[MWJB]

"A more accurate (and potentially more dangerous) way is with the amp switched off and discharged, to measure the resistance from each plate pin to the OT center tap, and then with the amp switched back on again carefully measure the voltage across these same two points, and then divide this voltage by the resistance you measured to get the plate current, and then multiply this plate current by the plate voltage-to-ground to get the plate dissipation." If you're going to go to the trouble of connecting up a meter accross the primary (CT to plate) of the OT with the amp live, you may as well set it to <200mA and actually measure the plate current (red lead to CT, black to plate), this is known as shunting the transformer. Don't do it with 5E? Fenders that have the plate & screen supplies tied together. Use crocodile clips/wire grabbers & make all connections whilst unplugged from the wall.

"Do bias meters take that into account, or do they "ballpark" it?" I have a TAD biasmaster that gives identical readings to shunting the OT, some bias probes may read fractionally high if they use a 1ohm cathode resistor to read current, either way you're really looking for a datum...as long as you continue to measure things the same way and evaluate by ear...you'll be alright. +/- 1 or 2mA isn't going to spell disaster unless you're really pushing your luck.

With JJ/RCA you could go up to 25-30mA, I don't know about the EHs either, one vendor advised me to keep them around 19-20mA.

[Bruce / Mission Amps]

Great Job!

I use JJ6V6s and older Russian GZ30s rectifier's in mine.
It makes about 18 watts clean and the power tubes are idling at 24ma-26ma with B+ at +430vdc.
Mine are built with 1 ohm @ 1% cathode resistors used as current shunt resistors so I can measure the idle current while beta testing.
If your bias tester is measuring cathode current (the safest way to do it), the additional idle current you are measuring from your 6V6 screen grids is probably minimal... maybe one or two miliamps.

The only real differences between mine and yours is that mine are made with an adjustable fixed bias supply, plus I'm using my Mission 5E3 power and output transformers, oh, and I wire my three input jacks differently.

I love these amps and think that, (with respect to the 5E3s) ...they are much better for actually playing out... especially with my cabinets all made up with their 12" speaker baffles!
By the way, the newest one I have out in the field right now has an Eminence, NEO magnet Lil' Texas in it and it is super loud... with tones set similarly, almost as loud as a stock Deluxe Reverb.
The next change is a Alnico ring magnet, Celestion Bulldog speaker clone.

[Josh_B]

Hi Bruce,
Thanks for the inspiration and the means to do this.
I have a little Bourns 50K 20-turn trimmer pot in that upper left corner for bias control.
If you don't mind sharing, I'd love to know how you wire your input jacks. I like the idea of the 1% 1 ohm cathode resistors. and bias check points I may try that, too.
Thank you again, sir!

[Bruce / Mission Amps]

Hi Bruce,
Thanks for the inspiration and the means to do this.
I have a little Bourns 50K 20-turn trimmer pot in that upper left corner for bias control.
If you don't mind sharing, I'd love to know how you wire your input jacks. I like the idea of the 1% 1 ohm cathode resistors. and bias check points I may try that, too.
Thank you again, sir!

You are welcome... I have a few others who also want to build one so I guess I'll just start putting something together.
Doh!... I didn't even notice the tiny blue box-bias pot!!
On my amps I use that space where your three 68K resistors are to mount some other caps and flying leads, which I can switch in and out by installing a little SPST mini switch in that hole next to your preamp tube.
Also, I've been messing with my the replacing the Vibrolux tone section with the one I loosely stole from the old Premier, Twin 8 amps (two 8" speakers with one 6L6s class A power tube).
They make decent low power harp amps so pushing it into the <20 watt range in the Vibrolux chassis has been pretty good for that extra gas the players always beg for.
Increasing or decreasing the idle current way down (hot'ish for guitar and colder for harp... I know, colder is backwards from what the Net harp amp consensus is) but with the right cap and resistor combinations it makes a rippin' harp and guitar amp tone stack too.
This new tone section is very similar to the one I did for webervst, the Mission Harp amp, the 5F2H.

[Josh_B]

I think you'd have a great offering with this one, Bruce. It's a very formidable gigging amp for small-to-medium clubs!