Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bullkelp on February 05, 2025, 12:40:14 pm
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Hi all
I have on my bench a bassman that appears to be a home build/kit of some sort.
It has the large mojo blackface bassman transformer. It was sold as a AB165 used on local the local FB marketplace
But I see it is:
Cathode biased
Has a presence control (on back)
Only has three pre-amp tubes
Has two large resavoir caps on board and a cap can mid chassis.
I wanted to check the bias as it smelt kinda hot when I played it (sounds pretty darn good bar one issue which ill get to)
I measure the cathode resistor as 330ohm
Voltage across it is 37v
There for current: 0.112 A for both tubes combined
Plate voltage 467v
Power = 52 w. / 2 = 26w per 6l6gc = 87% which I assume is ok given its cathode biased.
Issues:
1. Should I leave this cathode biased ? Im not used to seeing this in large watt amps. Is there a downside? Im assuming the PT will have a bias tap given its the mojo bassman PT
2. Im getting an unpleasant very high pitched tail note. Its glassy and ice picky and goes away almost completely with the presence down and the bright switch off. Is there a way to mitigate this so that the treble/bright switch/presence can still be used?
Thanks in advance
Ill post some images of the amp
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(https://i.imgur.com/IMh8JHx.jpg)
(https://i.imgur.com/nPOMOUk.jpg)
(https://i.imgur.com/AaUVjbZ.jpg)
(https://i.imgur.com/PstcyCp.jpg)
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by presence do you mean NFB?
unpleasant very high pitched tail note.
sounds like something is breaking into oscillation, scope will be you friend, as will a schematic
self-biased amps tend to get cranky when over-driven into class A/B, takes some tinkering to get it tamed down
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Yes I had assumed before opening the amp that the knob on the back is a presence control but you are right it appears to be a variable NFB
It is a 5k pot with a 27k resistor across it to the OT hot with a 0.1uf cap across the pot.
This runs back to a 0.1uf cap and 10k resistor that feed into pin 8 of the PI tube via the 470ohm resistor that is at the junction the two 1meg resitors per the ab165 . I cant quite see how this network takes the place of the 47k but maybe with some more study it will become clear
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for test, just "open" the circuit, so no NFB then check for your high-pitched noise
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So it would seem that it was most pronouced at the extreme travel of that pot. i.e less NFB
I cant detect a sound with NFB dialed back in to a reasonable level. Probably close to the schematic value.
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I have a mid 60s Bassman style amp and cascaded the V1 Gain stage into unused triode of V2, I had some issues with oscillation but cured with some snubbing caps:
V1 Plate to Cathode 47p snubber.
V1 Anode 100p snubber.
I have a presence circuit too, but it's a fixed value that jumps in a 4.7uf cap via push/pull master volume. Without it, I would get oscillation at high volumes, so pull it to mitigate.
5k sounds right for an NFB Control if the output is 4ohm like so many 60s Fenders. It probably has the 820r/100r NFB network off the 22k tail of the Phase Invertor, the the values are scaled down proportionally to your typical 8ohm or 16ohm "Marshall" type values we're more used to seeing.
Snubber caps works quite well too on the NFB resistor, even improves the tone in many cases if you're not using a 20ft cable imho