Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ginger on December 12, 2018, 12:31:08 pm
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Does 520V seem correct ?... that's what I'm getting.... seems way high to me... am I missing something , voltage doubler ?
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Sounds right to me.
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It seems about right. The schematic shows 500V and with today's wall voltages being higher that is likely correct. Is that your loaded or unloaded voltage?
The 1485 uses four windings stacked on each other, so it is pretty inefficient and will sag a bit under load. You can get slightly more power out of these by moving the screen tap to the 4th winding (same as where the center tap going to the OT that generates the plate voltage comes from) and add a filter cap stage, but with the really small OT it doesn't gain you much. Each 6L6GC pair will give you about 30 watts RMS instead of the 25 watts RMS they are stock with that mod. The 1484 was never a 50W RMS amp and the 1485 isn't a 100 W RMS amp either stock.
FYI, the 1484 schematic has an error where they show the bias supply grounded up by the grid resistors. The 1485 schematic is correct in that location and does not show the ground where it shouldn't be. Both schematics have an error at the trem tube. They have pins 6 and 8 reversed on the oscillator.
Greg
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Yeah... it is correct... and the amp sounds amazing... don't want to give it back... the trem is great... and the notorious bad verb is okay
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Yeah the trem is really nice on these...I think its because they inject the trem at an earlier stage. Nice sounding amps, though a PITA to work on and a poor layout.
Greg
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Yeah , it is a poor layout , and the reverb injects a bit of hum when switched on . How do I cool the bias on this one ?...with 5881s , I'm getting 42ma , I think I'd like to take it down to around 35ma
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No easy way. It's already running max cool. The easiest hard way would be to replace that simple bias supply with a voltage doubler bias supply (similar to the voltage doublers used on the B+). Then you could put a pot in the circuit and adjust for what you want. Are you up to the challenge and do you have enough room to do something like that?
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I have the schematic to do that.... I won't though.... no redplating . I'll leave it alone , I've spent MANY hours already , replacing blown 1/2 ohm screen resistors with 3 ohm , grounded power cord , adding individual speaker out jacks for the separate OTs , changing the taps to 4ohm, and installing internal F&T power supply caps instead of using a cap can
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Sounds like you've spent a fair amount of time on it...which on that amp takes twice as long to do the work as a normal amp due to the layout and construction techniques they used. As mentioned previously, a voltage doubler is how to get the bias circuit up to snuff. It isn't hard to do but takes more time and if the amp is working ok then why bother. Good move to go with the 4 ohm taps instead of the 2.6 ohm taps...a bit more common load.
Greg
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No easy way.
since I'm a contrarian by birth you could turn it to self biased I believe, 4 Cathode R's and tape off the -40. Will effect tone, and Audio power out some
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Sounds wonderful , for what it is.. and as you know , when you go to bed , thinking you've found the best amp... it's different in the morning