Which OPT: 4 x 6V6, cathode-biased (double deluxe)...

[jar240]

I want to build a guitar amp that will primarily use 4 x 6V6, but could handle 2 x 6L6. I'm thinking around 290V on the plates. Cathode-biased with P-P pairs having their own cathode resistors and bypass caps.

I'm trying to figure out the best output xformer to use, but I'm not sure.

From tube data sheets class A PushPull plate-plate impedance (pair):
- 6V6 8K, so 4 would be 4K?
- 6L6 5K

Should I go with a Hammond 1650N as the primary impedance is 4K4, even though it's power handling will be overkill?

In another post, dhuebert recommends using the 1650H for a modified BFA 4 x 6V6), but that's fixed bias. Would it work reasonably well under cathode-bias, 290V B+?

(BTW, I need a guitar amp more than a BFA right now, so the iron for that project will be sitting on the shelf for quite a while...let me know if anyone's interested in them, actually...they're brand new 278CX, 1650T, 159M)

Thanks,
Chris

[dhuebert]

For your output transformer, I would recommend:

TRANSFORMER, OUTPUT, 35 WATT, 4 OHM, REPLACEMENT FOR FENDER

P-TF22848

Replacement for Pro Reverb, Tremolux, Vibrolux, Bandmaster, and Vibroverb, 4 ohms. Equivalent to #125A6A & #022848.

Specifications
Audio Watts (RMS): 35
Primary Impedance (Ohms): 4,000 CT
Secondary Impedance (Ohms): 4
Mounting Centers: 3.5" vertical

From www.tubesandmore.com

and a Hammond 276X

Also; for cathode biased I would probably like close to 400 volts on the plate. The Hammertone Sixpac makes almost exactly 20 watts.


Don

[EWBrown]

Normally 6V6s operate into between 8K and 10K PP, for paralleled pairs just halve the OPT primary impedance:


4 6V6, in parallel push-pull, anything from 4K CT (common value is 4.3K P-P) to 6.6K P-P will work just fine, and will also work with a pair of PP 6L6GCs. 4.3K to 5K P-P would be the optimum OPT primary impedance range.

They'll run fine with a B+ around 285-300V, so 295 is spot on in the middle of those voltages. The cathode current can be between 35 and 40 mA each tube.

I recently built up a PPP 6AQ5 based amp (6AQ5 is quite similar to 6V6) and it uses 5K CT OPTs, made by UTK in Itlay. Nothing special, just cheap-grade 5K PP: 8 ohms OPTs.

Triode has some 4.2 or 4.3K PP "Fender" replacement guitar amp OPTs, those should work fine

For Hi-Fi use, I'd go with Hammond 1615s or better, for most guitar work the Triode OPTs or similar "Fender" replacement OPTs are more than good enough.

Sometimes some bargain priced Schumaker 4.3K 60W PP guitar amp OPTs show up on E-bay for pretty low prices, the seller had a "truck load" of them as surplus, couple years ago...

/ed B in NH

[Gingertube]

Guys,
I copied this from a post I made to another forum.

Note I used a 50W Marshall Output Tranny I had on the shelf but the Fender unit with Raa = 4K3 would probably be a better match.

This was a full "bells and whistles" amp which was power scaled, had cathode to fixed bias switching for each output pair and also had triode/pentode switching for each output pair. (It is a London Power "Standard" Amp from Kevin's TUT 5 with different Power and Output Trannies)

I also stuffed up the wiring to the Triode Pentode Switches - Where ever you see "pentode" mode below it was actually triode and vice-versa.

Copied Post:

Set to Work of the Power Amp.

Recall that I am using Hammond 370FX Power Tranny, 4 x 6V6G (ST Shape) Output Tubes and a 50W Marshall Output Transformer (Raa = 3K4).

Measured B+ unloaded (no tubes) => +378 Volts
Measured Raw Bias (no tubes) => -77 Volts

Set Power Scale Control to 10
Set Compression (SAG) Control to 0
Set Fixed Bias
Set pentode Mode

A quick calc for 70% of 12 Watts Anode dissipation gives 22mA per tube.

Fit the 4 x 6V6G Output Tubes and selected 12AX7 PI tube (selected for matched triode sections). Actually the best I had was a CV4004 so used that.

Set the 4 bias controls for 20mA per tube as a "start point".
Remeasure (loaded) B+, now at +349 Volts
Redo the calc above and get 24mA per tube.
Allow to warm up for 10 minutes and then set bias pots for 24mA per tube.

Checks:
B+ = 345 Volts
Bias Current/Voltages
24.0mA @ -27.1V
24.0mA @ -26.9V
24.0mA @ -26.6V
24.0mA @ -26.7V

Switched to both pairs to Triode Mode - each tube idle current drops about 0.3 to 0.4 mA
Switched both pairs back to Pentode Mode

Set Power Scale Control for B+ = 250V
Checks:
B+ = 250V
Bias Current/Voltages
16.5mA @ -19.5V
16.9mA @ -19.3V
16.6mA @ -19.1V
16.6mA @ -19.1V

Set Power Scale Control for B+ =150V
Checks
B+ = 150V
Bias Current and Voltages
9.8mA @ -11.3V
10.4mA @ -11.2V
9.7mA @ -11.1V
10.0mA @ -11.1V

Set Power Scale Control back to 10 => B+ back to +345 Volts

Wind Compression Control from 0 to 10 => B+ drops to +105V

Set Compression back to 0 and switch to Cathode Bias.

Checks:
B+ = 343V
Bias Currents / Voltages
Pair#1 Cathode Volts = +23.9V
25.9mA , 25.3mA tube currents
Pair #2 Cathode Volts = + 23.7V
25.2mA, 25.4mA tube currents

Note each output pair had a shared 470R 5W cathode bias resistor. No bypass caps were used.

Hope some of these measurements from my amp are useful.

Sonically a quad of 6V6 is a fair bit cleaner than a pair but still has plenty of that 6V6 "Grind" when cranked.

Cheers,
Ian

[cw]

IM sent.

i have an old 1966 Vox Berkeley head cab (looks like a Vox slant side Super Beatle) with a Peavey 30 watt (4-6BQ5's with 3-12AX7's with the reverb tank removed) that i would be willing to do a little horse trading with if you are interested.

the Peavey chassis fits quite nicely and replaces the old solid state that was there. it was all done before i got it, but the Vox serial plate is still there.

i can sell a NOS quad of RCA 6BQ5's and/or a quad of old NOS RFT General EL84's. i don't have a trio of 12AX7's sorry. i do have a matched pair of JAN PHILIPS NOS 12AX7WA/ECC803's i can sell. if you had a well balanced 12AX7 for a PI they would work very nicely.

the corners plastics are pretty bad on the head but can be had at the following link with all kinds of old Vox replacement grill cloth and original patterned Vox Tolex.

http://www.voxshowroom.com/northcoast/

personally i wouldn't do a total restore on the head cab, just some replacement corners and have the present (original) grill cloth re set and re glued. the mojo is perfect. the handle is good and all the plastic grills on the air vents are good. the white grill trim is a nice shaded ivory and the gold body trim has turned a dark bronze. i haven't cleaned the head/trim/grill cloth or tolex any as it would prolly wash out all the beer/dirt/sweat and tone! it will prolly be due for a re tube before long.

i am a bass player and need some BFA iron to go on my new chassis and into my Marshall look alike head cab i've ordered.

[EWBrown]

For a bass amp, you need at least one octave lower frequency reaponse (for the usual 4 stringer), and even lower for 5 or 6 strtingers...

Definitely the hi-fi grade iron like the Hammond 1650 series would perform far better at the low notes than would the standard "guitar amp" replacement OPTs, which don't really need t go under 80 Hz. They would probably sound somewhat "farty" in a bass amp Yellow_Light_Colorz_PDT_08

A Hammond 1615 would probably be OK for a small, lower-powered "practice" amp, 20 Watts or less.


/ed B in NH