Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on February 25, 2018, 02:19:39 am
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Hi guys, Just gathering some info on a PA conversion.
I believe the PA would be 100-150w, don't need that much power.
It has slide controls that are prone to problems so a complete makeover of the circuit is in order.
First off the power supply.
(https://i.imgur.com/yNZWkd8.jpg)
I have taken voltages with the taps disconnected from the circuit.
Top winding - 273v @350mAs
Bottom winding - 130v @100mAs
So my thoughts are to rectify the 273v to get approx. 360v (2xEL34)
This supply would also supply the screens via a 470ohm screen resistor.
The bottom winding will remain as a voltage doubler with screen supply removed and dropping resistors tweaked to suit for the preamp. Three preamp valves existing.
The OT is huge with a primary of 3.5k with 3.5,8 and 16ohm taps.
I'm thinking cathode bias but there is a bias supply.
Hoping for around 40w
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Why does slide-pot problems mean "complete make-over"?
If your house has a bad light switch, do you blow-out walls and put in new kitchen cabinets?
The proposed 360V is barely 40W in two EL34 with a very low load Z.
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Hey PRR, Probably didn't word it right. :help:
(https://i.imgur.com/CmJIymy.jpg)
These old amps had lots of problems with the slide controls and I don't believe a replacement is available these days.
(https://i.imgur.com/MRtGmw0.jpg)
(https://i.imgur.com/4BD6soc.jpg)
As for the circuit, the five channels are somewhat boring.
I have used this JTM60 preamp and sounds great.
(https://i.imgur.com/JI5tNIm.jpg)
I'll split the two channels and foot switch between them. :think1:
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I don't believe a replacement is available these days.
I fixed a couple 16ch mixers from the '80s and you're correct, those sliders were a bugger to source. I told the client to upgrade, he did :laugh:
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Tube amps should have rotary pots. Just get rotaries of same value and appropriate taper, plate-over all those slots and drill.
Values/tapers should be same-as conventional Fender tone/volume network.
I *could* see preserving the sliders, as it is a unique slice of guitar history. But not if it gets to be a drag.
I can agree with not having four inputs (5??) all the same. That looks like the Beach Boys' One Big Amp that they dragged to minor dates when they were starting out. Those days are gone. (But these drawings are this-century??)
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That really is a "180 Watt" amp. It falls just short of the 100w/pair extreme condition for EL34-- right near 800V supply.
Un-double the HV to get shy of 400V. It's a 45W amp with four EL34. May or may not be happy as 30W-40W with one pair.