I’m building something along the lines of the Ampeg Heritage B-15 Ampeg: Heritage Series - B-15 in a 3U rack-mounted chassis with Hammond 374BX, 1650 PA and 159P transformers. I have built one amp before, a stereo EL84 amp similar to a Leak stereo 20.
My main question concerns the bias and standby switches. The schematic I have is the first production model of the amp, with separate standby and bias switches. See here for a picture of the chassis, see the 3 switches on the right:
However they changed this mid-production to a combined 3-position switch with cathode bias up, standby in the middle and fixed bias down. See this picture:
I assume the reason they did this is because some people flicked the bias switch whilst standby was off, which would have momentarily biased the output at 0v with respect to cathode and probably made a bang. Is this right? or did they do it for another safety reason?
Can I make the same switch circuit as them? Using an ON-OFF-ON 4PDT switch rated at 380VAC @10A like this:
Wobble Lever 4PDT Locking ON-OFF-ON 3 Position 12 Terminal Toggle Switch | eBay
I would wire the two halves of the switch like the two separate switches in the schematic, with two poles of the switch between the power transformer and the rectifier, and the other two poles switching the bias. Here's a drawing to show you what I mean
The only thing I’m not sure is, when the switch is moved to the centre ‘standby’ position, there is no HT going to the rectifier but might there be still be some HT on the valves for a moment or so? Doesn’t it take a moment for the HT to dissipate from the power supply after the power is disconnected? This whilst the 6L6 cathodes are not connected to anything (ground nor cathode bias). But that should be fine right, no current will flow through the valve. The 6L6 cathodes are disconnected from everything at the same time as the AC is pulled from the rectifier.
Alternatively, if you think there would be no problem with switching between cathode and fixed bias with the HT applied, I would be happy to do away with the standby switch and just have a DPDT bias switch in its place. With the 12 second warm up time with the GZ34 on my EL84 stereo amp I have had no issues without a standby switch.
My other planned modifications to the circuit are: moving the HT fuse to between the power transformer and standby switch, having only one input jack socket with a channel switch and a pad switch, a 120/230 mains voltage switch on the back, and a Jensen JT-DB-E balancing transformer and XLR out with ground lift switch for preamp out/power amp in as well as the jack socket they have on the schematic. Any help would be much appreciated!
Schem:
My main question concerns the bias and standby switches. The schematic I have is the first production model of the amp, with separate standby and bias switches. See here for a picture of the chassis, see the 3 switches on the right:
However they changed this mid-production to a combined 3-position switch with cathode bias up, standby in the middle and fixed bias down. See this picture:
An externally hosted image should be here but it was not working when we last tested it.
I assume the reason they did this is because some people flicked the bias switch whilst standby was off, which would have momentarily biased the output at 0v with respect to cathode and probably made a bang. Is this right? or did they do it for another safety reason?
Can I make the same switch circuit as them? Using an ON-OFF-ON 4PDT switch rated at 380VAC @10A like this:
Wobble Lever 4PDT Locking ON-OFF-ON 3 Position 12 Terminal Toggle Switch | eBay
I would wire the two halves of the switch like the two separate switches in the schematic, with two poles of the switch between the power transformer and the rectifier, and the other two poles switching the bias. Here's a drawing to show you what I mean
An externally hosted image should be here but it was not working when we last tested it.
The only thing I’m not sure is, when the switch is moved to the centre ‘standby’ position, there is no HT going to the rectifier but might there be still be some HT on the valves for a moment or so? Doesn’t it take a moment for the HT to dissipate from the power supply after the power is disconnected? This whilst the 6L6 cathodes are not connected to anything (ground nor cathode bias). But that should be fine right, no current will flow through the valve. The 6L6 cathodes are disconnected from everything at the same time as the AC is pulled from the rectifier.
Alternatively, if you think there would be no problem with switching between cathode and fixed bias with the HT applied, I would be happy to do away with the standby switch and just have a DPDT bias switch in its place. With the 12 second warm up time with the GZ34 on my EL84 stereo amp I have had no issues without a standby switch.
My other planned modifications to the circuit are: moving the HT fuse to between the power transformer and standby switch, having only one input jack socket with a channel switch and a pad switch, a 120/230 mains voltage switch on the back, and a Jensen JT-DB-E balancing transformer and XLR out with ground lift switch for preamp out/power amp in as well as the jack socket they have on the schematic. Any help would be much appreciated!
Schem:
An externally hosted image should be here but it was not working when we last tested it.
Having the bias turn on when B+ is applied is going to be hard on the output tubes. Bias supplies take a few seconds to come up and stabilize. Bias needs to be running strong when B+ is applied, so I'd either use 2 switches or have the switch you currently have activate a time delay relay that controls B+
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Sorry, I was looking at your picture, the 3 way switch which is NOT ok.
Ampeg's schematic is ok with separate switches for the bias (on-on) and standby. Your 3 way switch drawing is not good because it has the problem described in my first post.
Bias needs to be running and stable before HV is turned on. Your 3 way switch does not do that.
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