5F4 for harp - ideas?

[Bluefinger]

Hi,

I could use some fresh ideas. I have a 5F4 that I have been using for guitar but now I want to use it for harp. It is already quite nice but I can't leave well enough alone so here's what I have so far:

More or less standarf 5F4 circuit with the exception of a .1 coupling cap on the normal channel, no bright cap and two 33µF filter caps (PI still has a 16µF).
Speakers are Weber 10A125.
Tubes are 12AY7, 12AX7, 12AX7, 5881, 5U4GB

What I would like to achieve:
A little more feedback resistance
Tight bass and good bite without loosing warmth

What I have tried so far:
o) I got a tighter bass response by going up with the filter caps but the creamyness (nice wurd, huh?) suffered a bit.
o) I tried 6L6 but they don't break up as smooth as the 5881s.
o) I tried reducing the 4.7M -FB to 10M and even no -FB which gave me more punch and breakup but less feedback resistance.

I haven't tried dropping the preamp voltage yet ...

any suggestions for that speciffic amp?

thanks!!!


EDIT:
I have done a little research on this forum and some seem to like cathode biasing. I have found directions to a conversion for a 5E3 here 5E3/6L6's dual Bias Switch. I know how to select the correct values for the fixed bias section but have no experience with cathode bias. Since this diagram is for a 2x 6L6 amp, how would I determine the values for a Tweed Super with 5881 power tubes?

Bruce Collins had mentioned that he likes his 5E7 with cathode bias and a Super Reverb OT for Harp. I assume the OT change does not require any further modifications ... am I right about this?

thanks again!!!

[Ronsonic]

Here's one I'd try, on the PI stage, go up on the plate resistor to give an asymmetric drive to the output tubes. Experiment from the stock 56K up to 82K maybe. I haven't done it so can't make any promises but should be interesting. The effect will depend a bit on your reduction of NFB, since NFB would work to negate the effect.

[MWJB]

"I know how to select the correct values for the fixed bias section but have no experience with cathode bias. Since this diagram is for a 2x 6L6 amp, how would I determine the values for a Tweed Super with 5881 power tubes?"

If your plate voltage (measured to ground) is in the 350-370v region, then try a 250ohm 10W cathode resistor, 330ohms will work up to around 460vdc, 470ohms for 450-490vdc, 600ohms for anything over that. Basically, I'd be looking for around 60mA per tube at the lower end of the voltage scale up to 50mA per tube at the higher.

If you do drop the preamp voltage (try 56K or 82K @ 3W instead of the stock 10K preamp dropper, aim for 90-120vdc on the 12AY plates?) you may find that you can use a tighter rectifier and still keep warmth, especially in cathode bias. The tighter rectifier will help bass & bite, I nearly always prefer solid state in a stage harp amp.

[Bluefinger]

Thanks!

From what I have researched meanwhile, the 250ohms resistor is what was used in all the old cathode bias tweed layouts as well. I'll start with that and see where it leads me.

I have tried a couple of values, up to a 100k for the dropper so far. I don't remember the exact readings but the voltages went down pretty dramatically with the 100K. I decided that I did not need the additional breakup and went back to stock. I might try it again with tighter filtering tho. So far I went up to two 33µF caps instead of the stock 16µF. It helped a bit with the bottom end. The third filter cap for the PI remained unchanged since I don't think it would help any. How far would you go up with the filter cap values?

I have never tried a solid state rectifyer before. I think I will get me one of those plug in solid state gadgets, so I can go back to a tube without a mod.

The problem with this is that I want to keep it somehow useable for guitar, when the Pro is too big for a certain venue. I am planning to build me a Bandmaster that will be used for harp exclusively. Then I won't have to make any compromises anymore ..

I am running around 370 Volts indeed but at 60mA the 5881 23 Watts tube would be running at almost 100% at idle ... is that ok in a cathode biased amp?

[MWJB]

The 2 filters either side of the choke (PI filter) are not going to greatly influence things even if you make them bigger, unless you rewire the choke to sit between B+ & screen nodes, as on a later Fenders. Use discrete 470ohm screen grid resistors, like on a BF/SF amp. With the later choke wiring 100uf will be fine right after the rectifier, 40-50uf for the screens, 20uf for the PI, you could try 20-47uf for the preamp and go by ear.

The 250 cathode resistor on old tweeds usually ends up at 300-330ohms once you have allowed for the rise in wall AC, unless you know that you are only going to get <370vdc I'd start with 330ohms (obviously your voltage will rise with a tighter rectifier), this will cover the broader range of voltages that you are likely to find in the amp.

If you keep the preamp voltages reasonably stock & the bypass & coupling caps, then the amp should work fine for guitar as well as harp (though maybe not simultaneously because of the shared tone stack). 2x10" tend to be more flexible in this respect.

From that 370v you will deduct the cathode voltage, let's guess at at least 25v, so 0.060* 345= 20.7W per tube, they should take it. What brand/age of 5881 are you using? The Sovteks should handle 22-23W dissipation no problem, TAD claim 30W for their 6L6WGC so aim for about the same with them. In short, I'd treat any current production 5881 as a 25W tube at least...if you're using New Jersey Tung Sols, I'd err on the side of caution just because of the cost of a new pair.

I wouldn't give up on the dropped preamp voltage but I would make sure that you try it with the stiffer rectifiers before making your final verdict. 100K preamp dropper when you are starting with 370v after the rectifier might be overkill.

[Bluefinger]

The 2 filters either side of the choke (PI filter) are not going to greatly influence things even if you make them bigger, unless you rewire the choke to sit between B+ & screen nodes, as on a later Fenders. Use discrete 470ohm screen grid resistors, like on a BF/SF amp. With the later choke wiring 100uf will be fine right after the rectifier, 40-50uf for the screens, 20uf for the PI, you could try 20-47uf for the preamp and go by ear.

The choke on a 5F4 is already wired between B+ (coming off the standby swith) and the screen nodes (pin 4) so I guess this should work. I would not have dared to go this high with the filter caps. The change I achieved with the 33µF caps was rather subtle. I guess this will be more audible than my pathetic approach

The 250 cathode resistor on old tweeds usually ends up at 300-330ohms once you have allowed for the rise in wall AC, unless you know that you are only going to get <370vdc I'd start with 330ohms (obviously your voltage will rise with a tighter rectifier), this will cover the broader range of voltages that you are likely to find in the amp.

ok ... thanks for the heads up

If you keep the preamp voltages reasonably stock & the bypass & coupling caps, then the amp should work fine for guitar as well as harp (though maybe not simultaneously because of the shared tone stack). 2x10" tend to be more flexible in this respect.

I'm not going to use it simultaneously anyway. On regular stages I usually plug the guitar into my Pro (which is a killer harp amp also BTW) and the harp into the Super. On smaller stages I use the Super for guitar and a Pro Jr. for the harp.

From that 370v you will deduct the cathode voltage, let's guess at at least 25v, so 0.060* 345= 20.7W per tube, they should take it. What brand/age of 5881 are you using? The Sovteks should handle 22-23W dissipation no problem, TAD claim 30W for their 6L6WGC so aim for about the same with them. In short, I'd treat any current production 5881 as a 25W tube at least...if you're using New Jersey Tung Sols, I'd err on the side of caution just because of the cost of a new pair.

I am using new production Tung Sol 5881s. These are my favorite 6L6 type tubes, followed closely by SED winged C 6L6 if I need more output power. The Tung Sols are rated at 23 Watts so I gotta be careful. I always forget to deduct the cathode voltage ... stupid me

I wouldn't give up on the dropped preamp voltage but I would make sure that you try it with the stiffer rectifiers before making your final verdict. 100K preamp dropper when you are starting with 370v after the rectifier might be overkill.

Yes, the 100k resistors were too much but below that I did not hear a whole lot of a difference since there is already a lot of breakup at regular gigging volumes. Let's wait for the bigger filter caps ...

That was a lot of great stuff. I am soaking this all up like a sponge but with every new thing I learn, I run into ten others I have no clue about. WHat a great place ... I really appreciate all that info!!!