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Re: It works!! (now some questions)

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1/13/1999 8:39 PM

Re: It works!! (now some questions)

MJ:

    The "Boost" switch changes the value of the treble cap/slope resistor from 330pF/56k to 1010pF/33k. So it is like the "Shift" switch on an older Mesa Boogie amp...

    The "Full/Warm" switch is just a Ck switch for the initial preamp stage: 22uF for "Full" and 0.68uF for "Warm". Incidentally, both switches are slide switches; if you used a typical toggle switch the contacts would be flipped upside down. (With a center-off toggle switch, you could add a third choice, "Clean", with no Ck for lower gain and more headroom.)

    Since the SS rectifiers are already there I figured that I'd add in the option to use them... As many recent posts here have mentioned, lowering the B+ to the output tubes doesn't really change the sound of the amp that much. I still need to redesign the power supply a bit and I'm thinking about adding a switch to select a lower B+ for the driver/PI tube (which evidentally has a greater effect on the sound, response, and output level of the amp.)

Steve Ahola

1/13/1999 9:19 PM

Re: Tweed Bassman mods

Actually I was going to use the zener to drop the overall B+ voltage so that the output tubes, PI, and preamp would all be operating at more "vintage" voltages. I'll have to explore the option a bit more. I've got the room in the case.

Right now I'm toying with the idea of putting in a tube driven fx loop; while at the moment I don't actually have an fx processor it could be a useful feature especially to add a touch of reverb which the amp could use. However, it would require either running the signal lines back over the whole circuit (using shielded cable of course) to get to the tubes and jack locations (potentially more noise problems) -OR- I have to shift the whole circuit board to the left and rewire all the sockets and pots which would be a major pain. I also would have to add the aux. filament transformer back since I disconnected it when I scrapped the idea of adding a 2nd channel to the amp. All that sounds like a major hassle.

I put the amp back in its cabinet last night and dragged it off to band practice to give it a try. The results? It worked perfectly. The overdriven sound sounds very good, the clean good (a little cold but I need to increase the bias current a bit; it's running at 33mA which is a bit cold for the EL34's in there; I also think a bit of reverb would improve the clean sound). The hum is quite a bit less noticeable in the box so it may be partly the result of environmental noise; I use single coils most of the time anyways so a bit of hum is unavoidable no matter what I do. I've very pleased with my project so far though...

1/11/1999 6:17 AM
Dave H	Re: It works!! (now some questions) The B+ on my 5F6A is 495V. I temporarily bodged in a chain of zeners to hear what the amp sounded like with the "correct" B+. It distorted a little earlier but sounded much the same to me so I put it back to 495V. Dave

1/14/1999 9:10 AM
MJ Harnish	Did you change the bias setting after changing the voltage? I'm just wondering since if you left the bias the same the tubes were running quite a bit colder than at the higher voltage and that could account for the lack of difference between them. Just a theory... MJ

1/14/1999 10:42 AM
Dave H	Yes, I set it to 35mA for both B+ voltages. Dave

1/14/1999 2:08 PM

Did you try setting the bias current any higher at the lower plate voltage? The reason I ask is that theoretically your tubes were running quite a bit colder at the lower plate voltages at 35mA which should have made some difference (I'm not sure how much).

E.g.:
495v * .035A = 17.3W
450v * .035A = 15.75W
435v * .035A = 15.2W

So, to have them running at the equivalent dissipation you would need to up the bias current to about 37mA at 450v or 40mA at 435v.

BTW - I'm not doubting your report of not hearing a difference; I'm just trying to figure out whether I should waste the $9 on a 22w zener... :O)

MJ

1/15/1999 8:04 AM

It’s OK to doubt my report. Cloth eared Harris may not be the best person to give an opinion on the subtle nuances of guitar amp tone. In a band situation, unless I’m actually playing the amp myself and can "feel" it, I can hardly tell the difference between tube and SS !

I only compared the amp at 450 and 495V at the one bias current not at the same dissipation. I set the bias at 35mA because at 395V I couldn’t detect much tone change between 30 and 40mA.

You could bodge in a couple of resistors to lower the B+ and only spring for the zener if you prefer the sound. There’s no substitute for hearing it for yourself. Try putting about 220 ohm 5W in series with each plate of the GZ34.

Dave H.

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