5F6 phase inverter and cathode bias?

[Bluefinger]

... would that combination work? I am thinking about using this in a harp amp project.

Would disconnecting the bias supply, grounding the common point of the two 220K resistors and putting a resistor/cap between ground and pins 8 of the 6L6 tubes really be all it takes? Any things to watch out for?

thanks a lot!!!!

BF

[txstrat]

Just watch to NOT choose the cathode resistor too small, cause the tubes might redplate. Look at the Fender Pro 5E5 schematic.
I'd start with a higher value cathode resistor (maybe around 350 ohms?).

[Bluefinger]

Thanks a lot!

Would you think that stock Bassman tranny would work with cathode bias or should I use lower b+?

[MWJB]

Make the bias switchable between fixed & cathode, going to cathode only will reduce output & make the amp less useful. If your B+ is higher than say 460-470v (at reasonable idle current) I would even recommend more than 350ohms, say 470-680ohms @ 10W. I doubt that you will need a cathode bypass cap on the power tube cathode resistor (increased feedback), you could switch it in/out. If you desperately want to stick to high idle currents/cathode bias only perhaps look at a smaller speaker array like 2x10" arranged vertically?

I'd use the later presence control with the 25K pot & 4.7K load resistor (see 6G12A & RI 59 Bassman schems), but keep the earlier 10K PI tail resistor (rather than 6.8K).

[MWJB]

"Would you think that stock Bassman tranny would work with cathode bias or should I use lower b+?" Are yuo buildig a giggable amp or a recording/lower power amp? A typical 5F6A tranny works fine for cathode bias, you just need to tailer cathode resistor value to suit, if your bassman PT is higher voltage (<525vdc) then 680ohms will work (<40mA), if between 460-490-ish then 500ohms (40-45mA?), if less than 460/470vdc then 330ohms will work (<55mA?).

If you use lower B+ the amp will be quieter...in cathode bias it will already be considerably quieter than a regular fixed bias 5F6A. If you did want to go lower B+, with typical 5F6A current ratings, then you might like to try 4x6V6, rather than 2x6L6?

[Bluefinger]

Thanks MWJB!

Well, I am definitely not desperately trying to stick to an idea. I want to use what makes sense and do the math first. Chassis, a buch of pots, switches, knobs, fiberglass board and other stuff is already ordered, trannies, tubes and speakers are still missing. I am a bit torn between cathode and fixed, because I want it to be loud but still love that cathode biased singing quality. It definitely is supposed to be a gigging amp. I guess switchable is the way to go.

Also I still have the idea in my head of using 1x12 + 2x8 Ohms Speakers. Maybe an 8 Ohms Cannabis Rex plus two 16 Ohms Weber 8A125-O. I have no idea if that will sound anythink like I think it will

[MWJB]

My preference would be to mix the 8"s with 10"s...this may still cut outright volume a little when compared to a 4x10" in fixed mode...if it was me, I'd stick to 4x10, perhaps pull 2x10" on one side when in cathode bias mode?

[Bluefinger]

I'd like it to be a bit smaller than a Bassman and a little less volume is ok so I am considering differnt speaker configurations ... 4x8 is a possibility too. Those Bassmans most local harp players are using are sometimes way too loud ... I know they have volume knobs but you know how it is )

[MWJB]

4x8 is only 200" sq., compared to 314" sq. for the 4x10"...I like 4x8" for recording, even for gigging with a couple of 10" to assist...but I think that for anything over quieter/moderate gigs & recording, 8" alone will strangle the amp. 3x10" is a little more "sq. than 4x8" but cuts better (if not quite as loud as 4x10")...but won't make a big impact on the size...in fact, not much will because of the chassis width - if you go 2x10" arranged horizontally you can get a nice compact cab, but horizontal 2x10" are notoriously hard to hear on stage & are therefore difficult to recommend.

Harp players have struggled to get decent stage volume for so long, that to many the concept of having a loud amp & playing quietly is a bit alien ;-). Plus for harp, you simply don't have the same useable range on the volume control as you do with a strat, it's all over & done within 10degrees of pot rotation ....dull, dull, OK, screaming feedback! A resistor tacked accross vol pot wiper to ground can really help get a decent sweep for harp, but you don't really want the knob pinting much past 6-7 at max volume before feedback, too much reduction in series resistance in the unused portion of the pot can lead to more highs & increased feedback.

[Bluefinger]

Well, the chassis I am going to use is 17,5 wide so I can reduce the cab width to the size of two 8" speakers side by side. Maybe this is going to be a bit small and will sound a bit boxy, so I was thinking about that 12" in the bottom and the two 8" on top of it which would result in a larger (higher) cab as well. That was the initial idea. The tweed Bandmaster combo (3x10) is an option as well. The more I think of it, the more confused I am.
The basic idea is to build an amp that is a bit more compact than a Bassman but still can cut it in a live situation with a moderate retro blues band (drums, double bass, piano, 30 watts Tweed guitar amps and such ...)

[MWJB]

It's not going to be easy to fit the circuit into a 17.5" chassis, you'll most likely need to mount tone caps on the pots, mount 68K input resistors on the jacks & use a filter cap pan outside of the main chassis.

[Bluefinger]

I am planning to use an external cap can and by using the 5F4 preamp section I will save a little space too. That bias circuit on an original 5F6 board is spread out very generously as well. It should work if my calculation is correct. If it doesn't I'll use a 20" ... that will do the trick.

[txstrat]

I've used the 5F4 preamp with a 6G11 PI and it's my main amp ever since. Great breakup and enough headroom (with two 10'' Jensen C10Q ceramics) for every club (at least for guitar). I'm using a 35w Vibrolux OT and two Tung Sol 6L6.
I really dig the 2x10 configuration. The 10'' Jensen ceramics are quite loud and their response is very tight. I like that.

[Bluefinger]

Sounds good. My favorite guitar amp is a self built Tweed Super as well. It is pretty close to stock specs tho. I'd like to mod the hell out of it, but unfortunately it already is perfect I never ran out of volume with that amp as well.

How did you connect the 6G11 PI to the 5F4 preamp? I guess I just have to connect the wire coming off the wiper of the 5F4 treble pot to the .022µF cap that connects to pin 2 of V3 and the 1Meg resistor on the other end. Is that correct?

thanks!

[txstrat]

How did you connect the 6G11 PI to the 5F4 preamp? I guess I just have to connect the wire coming off the wiper of the 5F4 treble pot to the .022µF cap that connects to pin 2 of V3 and the 1Meg resistor on the other end. Is that correct? thanks!

I'd put the trebe pots wiper to the .022 cap. Actually I have a reverb circuit in between the treble pot and the PI (with a 100K resistor).
Look on the pic.