two problems with tweed style build

[Bluefinger]

Hi,

Maybe someone can help me with this. The amp I am building is configured this way: 5F4 preamp with added mid pot (like Weber 5E7M), PI and power section mostly like 5F6A, added fixed/cathode bias switch. The amp is dead quiet and sounds fine but there are two problems:

1) only in fixed bias mode, and when I send a strong signal throug it it sounds like it's switched on and off rapidly ... like a machine gun. Does this make sense? Like a tremolo but a lot harsher. Is this oscillation? What can be the reason for this?

2) The mid pot has no function. It's wired like this: https://taweber.powweb.com/store/5e7_layout.jpg . I tripple checked it but the wiring seems to be correct ... still it's absolutely useless. Has anybody tried this tone stack yet?

many thanks in advance!!!!

[Jazz P Bass]

That tone stack "looks" Fenderish.
A schematic would be nice.
You really should check your work as this seems like it is to the original 5E7.
As to the fixed bias, recheck that the output tube cathodes are indeed at ground potential when the switch in engaged.

[Bluefinger]

Hi,

here's the schematic: https://taweber.powweb.com/store/5e7m_schem.jpg

the cathodes read 1 ohm to ground in fixed mode (because I have installed bias test points) ... I'll post pics of the wiring later. maybe that helps ...

thanks!

[Bluefinger]

Here are some pics ... maybe that helps:





thanks!

[Steve Conner]

Very nicely built!

The "machine gun" effect sounds like some kind of motorboating. Does it only happen with strong signals? Or does it happen with all signals, but strong ones just make it more obvious? Where are R11 and R24 mounted?

The layout you linked has no mid pot. Did you mean presence?

[Bluefinger]

thanks!

sorry, I posted the wrong link indeed: https://taweber.powweb.com/store/5e7m_layout.jpg ... this should be the one

The "machine gun" seems to disappear completely with very low signals.

The layout and schematic above were just for the tone stack. The PI and power section are wired more or less like this:
https://taweber.powweb.com/store/5f6a_layout.jpg
https://taweber.powweb.com/store/5f6a_schem.jpg

The filter caps are bigger and there is a fixed/cathode switch but apart from that it's pretty close.

thanks!

[Bluefinger]

Something just dawned on me ... I am using a 5F6A style power section but with a 4 Ohm load instead of 2 Ohms. Shouldn't I use a different -FB resistor then? I know it's a matter of taste to a certain degree but could that be the reason for the motorboating?

[UniPlank84's]

It sounds to me that your possibly getting too much signal voltage at the peak of the swing and the power tubes are turning off, or whatever the correct terminology may be- such as changing class of OP from AB to B etc. I am not an engineer though and this is my hunch based on gathered bits of info and exp as a fellow self learner/scratch builder.

You have wedded a preamp section to a power section so you need to expect possible tinkering and trial/error if not engineer and math wizz. You might try splicing in a MV pot before the PI, then if that does it measure the split and install a voltage divider. Another trial I have found helpful are lower value coupling cap(S) between TS and PI, but that may be less specific to your situ. Are your voltages within known working/compatable ranges? Have you measured the signal swing voltages into the PI? My hunch is if you tame the peak voltage it will then operate. Just a guess hunch-I do the same thing and have now been thru many many circuits in K-bias and fixed with my P/P 6V6 learning amp and now have my own fav derivative. Is fun hobby for sure.

[Bluefinger]

Thanks ... these are some things to consider and experiment with.

I just meassured all the voltages. The PT put out a pretty high B+ so all voltages in the preamp section are up to 30 percent above specs. I get around 456 vdc on the plates and screens. I think that's ok for an amp thats supposed to be punchy and loud but V1 though v3 seem a bit high. On v2 I get 323 vdc on pin 6 for example. I think I will put in a higher value voltage dropping resistor in the preamp and might even add one between V2 and the PI to get the voltage on V1 and V2 even lower. Low plate voltages are more usefull in a harp amp IMHO. I'll report back when I have don ethis and the things you suggested ...

thanks!
BF

[loudthud]

Something just dawned on me ... I am using a 5F6A style power section but with a 4 Ohm load instead of 2 Ohms. Shouldn't I use a different -FB resistor then? I know it's a matter of taste to a certain degree but could that be the reason for the motorboating?

You are correct. The Fender 5F6A schematic shows a feedback resistor of 27K. Weber shows R18 is 56K ??? Try the amp with that resistor disconnected. The amp will have more gain but oscillation should stop. If that fixes it, increase the 56K to something around 75K to 100K.

[UniPlank84's]

Loudthuds suggestion may be it come to think of it. On one plan I've studied it shows about a 30-50% lower value for 2ohm from 4 - as in 22k from 39K specifically. Also getting the voltages down to the 415 'A' node and then on per the Fender Super scheme can't hurt - when all is good is a great classic amp - one that I wanted to try a few months ago in a tinkering mode after reading Tweed Super raves. I tried 5F4 preamp section into a 6v6 class A power but it didnot work well, and moved on back to simpler fare for may current parts stash. From my own exp'mntg I have found it seems best to make only one smallish section change and get it running and go from there, or revert to all stock, and try again, and over time some things start making sense for a semi-self taught.

It sounds like it is good in K-bias?...if so then your nearly there. A good quality 100k lin pot is good to have for feedback trials I hear like a PEC 2W or something. I smoked a 100k CF resistor once in a 6V6 jtm type thing, so reasoned it worth having a bit more capacity for the higher R values there maybe.

FWIW I have had good luck lowering my vintage 117v rated PT's supply that yeilds 350/0/350 w/ 124 wall V's down to its labeled 320/0/320 using a 2 bulb limiter in lue of a VariAC if you don't have one on hand. I assume this a non-standard practice, but have used it for 2 years now with no probs, but O'sMMV. Saw a nice quality step AC converter recently Ebay but missed the auction end-I think that type is ideal for continuous duty as opposed to VariAC-maybe. Currently this 2 bulb set-up has 100w and 60w combo. I just watch and keep the heaters in range/exact, and have found browning down some very useable tonewise @ 6.1v or so.

Yes-really nice work and build. When you get it going well should be a pleasure for many years.

[Bluefinger]

ok, I just removed the -fb resistor completely and while it did not cure the motorboating, it improved the sound greatly. On my close to stock 5F4 I switch off the global -fb when I play harp as well so I will keep it this way for now. It has a lot more punch now without suffering from increased feedback. What did help was to increase the 4k7 dropping resistor between the first two filter caps to 15K. The voltages are now within 5F6A specs. It's still not perfect but I'm getting there. The motorboating effect is now reduced to a single pop every once in a while as opposed to the pop-pop-pop-pop-pop thing that was happening before (sorry for the "very scientiffic" description). What I need now is simply a couple of hours to try some things to get this sorted out. I'm now quite there but I'm getting there ...

[tubeswell]

So what are your (DC) idle voltages (HT, Plates, screens, cathodes) throughout the amp? Can we see a schematic?

[Bluefinger]

The schematics I used as a starting point are posted somewhere above ... preamp is mostly a 5E7, PI and power sections 5F6A. I'll write down the voltages next time I'm in the workshop. I might try to use a 12AT7 in the PI. Maybe that tames the signal a bit as well.

[MWJB]

Try the 12AT7 in the PI by all means, but the amp should work fine with 12AX7 in all 3 preamp slots (whether or not that's what sounds best to you).

I notice that you have omitted grid stopper resistors on the power tube pin 5s, you have no buss wire on the control panel (it's typical to ground pots & pot mounted components to the pot backs/buss wire, rather than to run the grounds down to the daisy chained grounds on the board) & it looks like you have one ground wire connecting preamp & power amp grounds. I would separate these, ground the preamp at the input jack end of the amp, ground the power amp at the PT end of the amp. Check all pots & jacks are nice & tight on the chassis.