Express Combo using 6V6s (Finished)

[UR12]

Dana - OK, trying to keep a consistant language here, the PI 'tail' in the Express and Fender BF amps refers to the resistance to ground that comes off the juncture of the PI grid and cathode resistors, i.e., the two 1 megs and the 470 ohm resistor.

So in part, this would be the 10K resistor on the Express, which is where the OT feedback is returned, and then the added resistance to ground - off of that - which is the 5K Presence pot. BTW, I think it is because there are two resistors here, in series, with the feedback voltage returning at a point between the two, that it is named the "long tailed pair". So the two resistors together are the actual 'tail'. And actually I think that if it is to be considered a true 'long tailed pair', there needs to be the OT feedback applied.

Translating then, the VoxAC30 is refered to at times as having a version of the 'long tailed PI' - but I don't think it can really be considered a true example because the negative feedback is not applied..

Anyway, to finish up, as far as I understand things, the 10K value can be increased with mainly the affect of balancing the PI and also reducing gain - and the higher value resisitor, say 47K would also reduce the negative feedback voltage from reaching the PI more than a 10K value, true enough. .........However, this being said, the ratio of NFB voltage is still determined by where it sits in relation to ground, and this is determind by the second resistor in the 'long tailed pair', which in this case is the 5K pot, (or the 47/100 ohm resistor in the BF Fender series.)

So when you determine the *ratio* of NFB in the PI circuit, you still need to divide the resistance coming off the OT secondary by the second resistor in the long tailed. Then given the constant first resisitor in the long tailed pair, you can start talking 'NFB ratios'.

I think a cat can be skinned many ways, true, and you can change that first resistor in the tail and it will affect things that involve the NFB. ...But for the sake of speaking and understanding things, there still is a ratio and it has nothing to do with the first resistor in the tail. Are we on the same page here?

Sorry if I am being confusing. I am talking about the resistor to ground that come off of the series of resistors connected to the cathode circuit of the PI. In your Fender example that would be the 100 ohm or 47 ohm. In a TW that would be the 5k presence pot. These are the only resistors I was refering to, not the 10k, 470, 1k or 39k in the TWs

[UR12]

Double post

[rooster]

Dana - OK, I'm reading you. And I do use the 25K pot + the 5K resistor in two of my amps. Eh, at one time I thought it was the best, now I don't know.

Anyway, your combo amps look very cool, BTW, and I particularily like your take on the chassis itself. Take care.

[rooster]

Dana et all - OK I did do some swapping of the NFB resistor. I first added 30K to the existing 100K, so 130K total. I was sure this was going to really help things. ...................But it didn't. In fact, it increased the already present yet rather slightly vague 'gritty/on the edge' sound - and this is hard to describe. Saying the 'grittiness' of the amp increased would be accurate, I suppose. I hope this translates.....

So then I followed Dana's direction by replacing the 100K with an 82K value. This means I have now tried three different ratios here. The highest, 26:1 (130K/5K), 20:1 (stock, 100K/5K), and now 16.4:1 (82K/5K). Dana is using an even smaller ratio, 13.6:1 (68K/5K), BTW.

Well, at this juncture to my ear, I think Dana is very correct to suggest reducing this ratio. I may never try the 68K value but I would venture to say that I won't return to the stock value either. Its the 'grittiness' factor I think. By increasing the negative feedback the amp sounds better, and not so 'gritty/out of control/on the edge'. Yeah, these are words trying to describe a sound, not easy. Believe me, I am saying that to my ear the amp sounds and behaves better having made this adjustment. I am not using the words 'gritty/on the edge/out of control' in a good way here, as pleasant an image as that my be for most of us....

Anyway, my take on this is that this is very worthwhile adjustment and something those of you building 6V6 Express's should really consider.

Dana, thank you for presenting your thoughts here. This has been a very worthwhile post. ...........Oh, my amp thanks you, too!