Feedback Resistor

[TIMBO]

In my build it has a 10k FB resistor and on the original schem it has it connected to the 16ohm tap of EL34 tubes PT. I have 6V6 tubes and only 4 and 8ohm taps with the 10k resistor connected to the 8ohm.I am getting ALOT of feedback at higher volumes. Is there a rule of thumb to the size of the resistor.

I took some ideas from Mark Huss's 6V6 plexi and he has a 100k resistor for his but the Hiwatt is different in that area. Some ideas, Thanks 

[Tone Junkie]

Timbo for the feedback resister it works like this the smaller the resistor the more negative feedback you get, a larger one the less negative feedback you have. For you more is probably good .
But there is a balancing act in there I dont understand. maybe Sluckey or PRR can chime in and explain .
In my marshall clones that could be from 47k to 100k or like in a splawn 220k on the feedback resister . I take a 250k pot and wire it in find the sweet spot and then switch it out for a resistor of the closest size.
Also changing which tap raises or lowers how the transformer see's it. so switching and trying it at the 4 ohm tap wont hurt. but you might put in a 25k pot and find the sweet spot that way.
Sorry Im not alot of help its late Im tired.
Bill

[tubeswell]

You don't want too much global feedback. Global NFB is driven from the OT secondary, and the signal phase lag between the OT primary and OT secondary is different for different frequencies, therefore too much NFB starts to progressively result in positive FB at certain (high) frequencies. So there is a bit of a balancing act.  What's the rule of thumb for the 'right' amount? That's up to your judgement and preference. The amount of global NFB (in most amps) is the product of the VAC at the OT secondary (different for different turn's ratios, tube types, load resistances), the strength of the voltage divider in the NFB loop, and the strength of the signal at the NFB insertion point in the signal path. So you have to suck it and see.

[SILVERGUN]

I take a 250k pot and wire it in find the sweet spot and then switch it out for a resistor of the closest size.

+1.....a great method for getting to the bottom of it

[TIMBO]

Thanks for that guys, put a 250k pot in place of the 10k andas i dial it up the feedback gets worse and it is best at no resistance  this also reduces some of the FIZZYNESS as well  Thanks

[jjasilli]

My vainglorious hope to provide an elegant and lucid insight was shot to hell by the complexity of the PI circuit fed by the NFB, not to mention the presence of the presence circuit on the other side.  All this appears to be interactive.  So yeah, plug in a pot; and, literally, play it by ear.!     Well done!

[tubeswell]

Thanks for that guys, put a 250k pot in place of the 10k andas i dial it up the feedback gets worse and it is best at no resistance  this also reduces some of the FIZZYNESS as well  Thanks

Are you sure you've got the pot input and wiper/output the right way around for the 'dialing'?

[Tone Junkie]

Timbo check your resistance across the pot and see what you have.

[TIMBO]

Hi guys, No luck with any thing that i tried.

I ended up removing the complete NFB/presence circuit and replacing it with a standard marshall NFB/presence and as did other circuit it had little effect and at high vol it would FB.

Swithed the OT wires but no change.

At the moment there is no NFB/presence circuit and it displays the same problem.

The amp is pretty quiet, its only when at about 9 out of 10 that it gets the FB and fizz.
The few thinks i have notice is that it is quite trebly and chimmy could that be the 6V6s and gets some of after note ringing and harmonics (i changed the preamp tubes but same)
With the foot switch open it adds some balls like a boost but different not distorted but meaner.Thanks

[jjasilli]

Other possible things to try:

1.  Increase the value of the NFB shunt resistor.

2.  Try a different speaker.

3.  Change the 47pF suppressor cap, following the PI, to 10pF or 100pF.

Maybe grid stoppers or local feedback on a preamp stage(s) will cure the problem.

*  Play another amp into that speaker.  Does it fizz now?

*  run signal from another amp's PI into the "Hiwatt's" power tubes.  Does it fizz now?

*  run signal from the PI of the "Hiwatt copy" into the other amp's > speaker

*  maybe try running signal out from various preamp stages, before the PI.

[SILVERGUN]

Timbo, just for the record,.,,I had a couple minutes today and I double checked the PI section of your schematic against the original Hiwatt version and it looks good.....

Have you given any thought to converting to LTP?.....then you could stick that "clipper" stage in there 

[sluckey]

Have you given any thought to converting to LTP?.....

It is a LTP, although fixed bias. But if converting to a more familiar LTP, you would also free up V3A. Now you have two triodes to play with.

[TIMBO]

I kinda really drop myself init when i build these amps, i see a unique way of building and it tweaks my interest and in the end it a lot of hard work to get it working.

I got a bit smart   on this one and made the connection between the components on top of the board, so to reroute thinigs is easier.
I have not had a build that has 6V6s in it so i don't know what to expect as an output sound???
I was thinking that i might use a pair of 6L6s and bias then to 24w as the pt is only rated at 25w and see what i get, but i'm just not sure if i have enough mAs in the PT

I do like the sound as is and could just not max it out......
Or keep the preamp and rebuild the PI, power amp to mark huss's 6V6 plexi.....
Or if it is still trebly,build the PI, power amp ot suit a pair of 6L6s.....  
The PT is an old bugger (1954) and the tube lineup was a guess at about 90-100mAs so the 6l6s may not be an option,any othe ideas for power tubes other than EL84s...... Thanks

TO BE CONTINUED  
  

[TIMBO]

Yeah sluckey, I tried a few values to adjust the voltage at the cathode of the PI and it didn't make much of a difference, so to change the PI i think is a goer. Thanks 

[SILVERGUN]

I hate getting caught sayin' dumb S#!+   .......thanks for always paying attention Sluckey

I'd love to see you free up V3 and try that clipper you mentioned....maybe you can filter out some of the treble with your cap choices there

[TIMBO]

Thanks SG, Could you point me to the caps that would give me that option, or any suggestions.

[SILVERGUN]

Here's what your last stage could look like...
Thats a great place for experimentaion.....
The grid leak resistor will "set" the gain going into that last stage, and I enjoyed using a 1M pot there to dial to taste...

In conjuction with that coupling cap, there is plenty of room for adjustment.....in fact the pic I used is from an earlier tweak session, and I have since settled on a .022 coupling cap, followed by a 1M grid leak resistor

Because it acts as a RC filter, the values effect each other,,,,so it's a back and forth game, and just depends on how much OD you're trying to achieve

You can also try adjusting the value of the coupling cap going into the PI.....you currently have .047 there, so just piggyback a .047 to get close to .1 and see how that sounds

[HotBluePlates]

Other possible things to try:

1.  Increase the value of the NFB shunt resistor.

That increases the negative feedback, and may make the problem worse.

At the moment there is no NFB/presence circuit and it displays the same problem.

Which tells me the problem isn't (only) the negative feedback circuit.

Very likely there is a wiring issue, where maybe a large a.c. voltage output wire is routed too close to a low-level high-impedance input earlier in the amp. OT plate wires near the phase inverter input would be a common way this might happen, though it could be wire routing at any point in the amp.