Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Jennings on February 28, 2023, 12:59:33 pm
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A month or so ago I resurrected a circa 1970 Fairview Music Stage Two 85W amp that seemed to be a bit of a prototype or home build. I'd certainly never seen another before. Thanks to the good folks here I reworked the bias circuit to a voltage doubler, and had it running much better than it probably ever did before. The amp's got a great tone and low noise floor...and seems to be predominantly a sort of single-channel Fender based amp, with beefed up twin KT88 output section. And yes, the emulation does seem to extend to the bird's nest build ethic it appears! Worth noting that although I've noted values of components, I haven't measured the pots as yet, and can't see stamped values on the back/sides. I'll measure in due course, though am assuming Fender-ish stock values.
I've got a few questions I'd like to pose to the collective wisdom here:
- When using the tremolo, the amp seems quieter in volume than when when the trem is off...is there a way to balance that up a bit, or is that just a factor of the design as it is?
- The tremolo depth control only seems to kick in noticeable trem after about one o'clock on the dial...so all the action seems to be in that last 1/3 of a turn. What's the best way to alter that...different pot value/taper?
- The reverb works well, but has lots of extraneous hiss/noise. Some of this down to the lack of room in the head-box and the placement with regards the rest of the amp, but I'm also wondering whether there are any tips for getting the unit quieter? Extra shielding and perhaps replacing the shielded cable runs?
- The last question I have is just a general one on the HT and standby switch design...what's the general opinion on adding an HT fuse? And on the Output mute version of the standby switch we see here?
Thanks in advance folks...and any typos/mistakes gratefully received for my schematic trace too.
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When using the tremolo, the amp seems quieter in volume than when when the trem is off...is there a way to balance that up a bit, or is that just a factor of the design as it is?
The tremolo depth control only seems to kick in noticeable trem after about one o'clock on the dial...so all the action seems to be in that last 1/3 of a turn. What's the best way to alter that...different pot value/taper?
That tremolo circuit works by shunting the signal to ground. It can only decrease volume, never boost it. Use a 50K-RA (https://www.tubesandmore.com/products/potentiometer-cts-50k-reverse-audio-solid-shaft) intensity pot.
BTW, your trem opto is draw wrong. It should connect to the plate, not the grid. And those two 22Ks connected to the footswitch probably should be 1M.
The reverb works well, but has lots of extraneous hiss/noise. Some of this down to the lack of room in the head-box and the placement with regards the rest of the amp, but I'm also wondering whether there are any tips for getting the unit quieter? Extra shielding and perhaps replacing the shielded cable runs?
Noisey, hummy reverb seems to be common these days. Most likely in the recovery circuit. Try a low noise tube such as 7025 or 5751.
BTW, that 270K reverb mix resistor is usually much bigger. Fender uses 3.3M.
The last question I have is just a general one on the HT and standby switch design...what's the general opinion on adding an HT fuse? And on the Output mute version of the standby switch we see here?
That standby switch should work just fine. I'm not opposed to a HT fuse on big amps, but I've never used one on the smaller amps I have.
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Thanks Steve…you’re absolutely right on my trem section drawing mistakes…and that reverb mix resistor too (another typo) 👍 I’ll correct accordingly, and try switching that depth pot too. Got a few 7025 and 12AX7 types so I’ll try rolling a few with a view to seeing if I can get the reverb noise down a bit 👍 I’ll keep the stand by as-is then, but maybe see if I can find a convenient spot for an HT fuse. Thanks for all the advice.
One more question I’m wondering about is around node B of the power supply. It uses four capacitors in a series / parallel arrangement, but how come it only employs balancing/bleeder resistors on one pair of the four? Physically the pair without resistors are located near the choke inside the chassis, and the other pair with resistors are inside the doghouse (presumably for space purposes).