Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Boots Deville on March 16, 2012, 09:05:17 am
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I had a spare Weber reverb unit PT, 260V non-CT secondary, 100mA that I thought I'd use in a budget 5E3 head build. I've built a couple 5E3's in the past, but both have found their way to new owners, and I miss having one around.
So here is my poor-man's 5E3(ish) amp:
Changes include:
- SS Rectifier
- .02 Coupling Caps in the preamp
- .05 Coupling Caps from the PI
- 1M Grid Stopper to the Cathodyne
- 22K Grid Stopper to triode before the Cathodyne
- 470R Screen Resistors
- Second dropping resistor in the power supply dropped from 22K to 10K to account for lower B+
- Single input jack with toggles to select Bright/Normal/Bridged input.
I used one of these PTs in a 6BM8 amp and got almost 360V B+, but in this amp I got a B+ of 340V and 328V on the 6V6 Plates.
I'm *really* happy with the way it turned out. For obvious reasons, I thought there might be noise issues. When I first powered it up, I had the red secondary wires running over the board to the rectifier diodes, and I had more hum than I would have liked. The hum level changed with volume/tone settings. Chop sticking indicated that moving those secondary wires away from the tone pot decreased the hum. I re-routed those wires under the turret board and now the hum is all but gone. Very low noise floor now and it sounds great, even without the 5Y3 sag. I might be getting some power supply sag. Sounds great to me.
One issue I'd like some input on. Instead of the four input jacks normally found on a Tweed Deluxe, I only included one, and added to toggles to be able to select between Bright Chanel, Normal Channel or Bridged (both channels). These work great, except there is a pop when switching either one. I've attached the schematic for this. Any ideas where I might strap a 10M resistor or two to stop the pop?
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1M or 10M resistor goes across ea pair of SW lugs: with the SW ON, the resistor is shorted and signal passes through the SW terminals around the resistor. With the SW OFF, the resistor is technically making a connection, but it's high value fully attenuates signal. This is often good enough to kill SW pop.
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Thanks. So you're saying with a DPDT and a SPDT I'd need six of these resistors. Being mini-toggles with tiny lugs and without much real estate, I was hoping to strategically reduce this number, but maybe there isn't a way.
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I think he means 2 for the DPDT and 1 for the SPST.
Real nice looking build!
Brad :icon_biggrin:
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I think that's a SPDT SW. Let's work on that SW first. Maybe try a R across the bright & normal lugs. If that doesn't kill pop, or leaks signal; then try 2 R's: one R from Bright Lug > Center Lug; other R from Normal Lug > Center Lug.
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I think that's a SPDT SW. Let's work on that SW first. Maybe try a R across the bright & normal lugs. If that doesn't kill pop, or leaks signal; then try 2 R's: one R from Bright Lug > Center Lug; other R from Normal Lug > Center Lug.
Thanks, but I'm a little unclear. The SPDT switch is the Single/Bridged switch, not the Bright/Normal switch. But I'll try that on the Single/Bridged switch - large resister across the outside lugs. If that doesn't do it, two resistors connected to the center lug, one to each outside lug.
She's in the cab right now for a gig tonight, but tomorrow I'll get her back on the bench.
Cheers!
-John
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Cute little amp!
I think if you just add a 1m to each grid to ground you will be good to go.
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Cute little amp!
I think if you just add a 1m to each grid to ground you will be good to go.
Thanks Dan. I tried clipping in a 10M from grid to ground, but that didn't do the trick. And I also only did one grid at a time "just to see...". Maybe the one left floating was still causing the pop. I'll try that with 1M on both grids.
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Yes, sorry, I meant the Single/Bridged SW.
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Very nice job
Look here how Vox do it in the AC30CC
Kagliostro
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That's what I'm talkin' about.
Love working on those amps... All those connectors and tracing out the circuit is just so much fun.