Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: liddokun on February 23, 2022, 09:02:25 am
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I have an AC15 build I've been tinkering with for some time. Amongst some of the things I've been trying to fix, one of the things I've noticed is the B+ being far too high.
It's built according to Sluckey's layout (with a a premade board I bought from here). No mods other than the ones listed on the Sluckey project (single switch to select either or both channels, rotary to choose brilliance caps, rate control).
I am using a pair of Classictone transformers, the ones spec'd specifically for AC15 builds. Secondaries according to datasheet for the PT is 305-0-305. I'm measuring around 385vac on startup on the secondary, and if left for a bit, it drifts upwards to around 400vac.
B+ at the first filter node sitting around 490vdc.
I tried pulling the rectifier tube and measuring the secondaries with the same result. What other tests should I be trying?
Incoming mains voltage is measured at 120vac. I know sometimes there can be a little bit of variation on voltage coming out of the secondaries, but this is a pretty big discrepancy. Is my PT a lost cause? Or is there something I'm missing.
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Are those voltages with EL84 fitted?
With no load current, they will be higher.
Heater winding voltage? With valves fitted.
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Transformer voltage spec are with load at 120 ma
If you load need less than 120 ma, voltage will rise.
https://s3.amazonaws.com/tubedepot-com-production/spree/attached_files/40-18079.pdf?1487782229
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Are those voltages with EL84 fitted?
With no load current, they will be higher.
Heater winding voltage? With valves fitted.
Yes, this is with EL84 fitted (also all other tubes fitted)
Heaters are seeing 3.7vac.
@latole Yes I thought about this, but is it reasonable for there to be such a huge difference?
I am using Sluckey's schematic for voltage references. I know there should be a difference in my readings as he's using a different PT with slightly different spec, but I'm getting almost 500V B+.
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My AC-15 draws 100mA from the rectifier. Measure the current draw of your amp. If you have a STBY switch this is easy. Set your meter to measure DC current. Use appropriate scale. Most meters require you to move the red probe to a different jack. (Don't get this wrong!) Turn the amp off and turn the STBY switch to standby mode (switch open). Now connect the red probe to the side of the STBY switch that is connected to the rectifier and connect the black probe to the other side of the STBY switch. Turn the amp on and your meter should read the total B+ current. What have you?
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I unfortunately do not have a STBY switch on this amp, so an extra step disconnecting the wire running off the rectifier to the first cap.
I'll measure the current tonight after work and report back.
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Is your mains voltage 120VAC? Is your PT the 40-18079? If yes and yes, are you using the black and brown primary wires?
Your voltages sound like what you would probably have if you connected the 120VAC mains voltage to the 100V primary tap, ie, WHT primary lead.
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Is your mains voltage 120VAC? Is your PT the 40-18079? If yes and yes, are you using the black and brown primary wires?
Your voltages sound like what you would probably have if you connected the 120VAC mains voltage to the 100V primary tap, ie, WHT primary lead.
Yes it's the 40-18079. This hadn't occurred to me before, I will double check which taps are connected.
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I can see where it would be very tempting to just grab the BLK and WHT primary leads. :icon_biggrin:
I'm hoping it's as simple as that. Otherwise, your voltages are just way too wrong. My Hammond 270FX, 550 CT @ 150ma, actually put out 604VAC when loaded with 100mA DC load. I wasn't too happy about that. But I'd be down right pissed if I had your numbers.
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So, I double checked the colour wires coming off my PT.
I am indeed using the brown and black secondaries.
I measure 79mA coming off the rectifier. I tried two different rectifier tubes with the same result. 79mA definitely seems low, which might explain the high B+.
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So I swapped the PT to a Hammond 270EX.
It's 550 ct @144mA.
I'm still getting uber high voltages. Close about 380vac coming out of the secondaries.
B+ is around 490vdc. B+ current draw is hovering around 101mA.
I thought there was an issue with my PT but it could be something in the circuit.
The amp works and sounds like it should, with the only peculiarity is a faint white noise or static sound when both volume pots are turned all the way down, and the top cut is turned all the way down. Turning any of these three pots causes a tiny pop sound and the noise disappears.
EDIT: I feel absolutely ridiculous. After getting totally frustrated over this, on a whim I remeasured my wall AC. Alarmingly, it read 159vac. The only time this ever happened before was when my multimeter was running low on battery. So I strapped a fresh one in and low and behold, all the weird high voltages corrected themselves.
Now I've got a spare classictone PT! Guess I'll save it for a dual lite project in the future.
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Yes, this is with EL84 fitted (also all other tubes fitted)
Heaters are seeing 3.7vac.
... I remeasured my wall AC. Alarmingly, it read 159vac. The only time this ever happened before was when my multimeter was running low on battery. So I strapped a fresh one in and low and behold, all the weird high voltages corrected themselves. ...
Your heater was 3.7vac from one side to ground, implying 7.4vac to the tube heater pins. It would have been smart to stop there & question the voltage that's 117% of normal. You might have noticed that crazy outlet voltage reading sooner.
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Your heater was 3.7vac from one side to ground, implying 7.4vac to the tube heater pins. It would have been smart to stop there & question the voltage that's 117% of normal. You might have noticed that crazy outlet voltage reading sooner.
You're right, unfortunately I decided not to. Indeed I did question it, but not seriously enough.
At any rate, I'm glad I have this sorted. If I can just figure out the noise when both volumes and top cut knob are all rolled off, I can finally box this amp up.
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Some high resolution pics of the amp circuit, voltage readings from all tube pins, does help others to help you!
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Some high resolution pics of the amp circuit, voltage readings from all tube pins, does help others to help you!
Yes, I'll post these up soon as I'm back from work!
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So these are the measurements I'm working with:
V1-12ax7 preamp
1-162v
6-215v
3-1.05v
8-19.1v
V2-EF86 preamp
1-115v
3-2v
6-96v
V4-12au7 modulator
1-86
6-83
3/8-3.5v
V5-12ax7 PI
1 220v
6 240v
8/3-61v
V6-12ax7 trem oscillator/PI
1-129v
6-217v
3-1.5v
8-17.8v
V7/V8-EL84
3-11.9v
7-338v
9-340v
Attached some photos. The "noise", which sounds like very low level static, only appears when both volume knobs are turned all the way off, and the top cut is all the way in the cut direction. If any of those three pots are turned, there's a faint pop and the noise goes away. Additionally, an alligator clip clipped to either of the top cut pot's terminals makes the noise go away (the other end of the alligator clip is not connected to anything).
This still baffles me.
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So these are the measurements I'm working with:
… The "noise", which sounds like very low level static, only appears when both volume knobs are turned all the way off, and the top cut is all the way in the cut direction. If any of those three pots are turned, there's a faint pop and the noise goes away. …
Schematic? V1 pin8 voltage looks rather high?
Those symptoms may indicate oscillation, with the ‘pop’ being due to a change in DC levels depending on whether it’s oscillating or not.
What does a scope on the speaker output show?
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I built off of the redrawn schematic posted by Sluckey, attached.
I unfortunately don't have access to an oscilloscope to check.
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Update: I didn't consider this symptom before you mentioned it, but I decided to work through some of the things to try when oscillation is a problem.
Adding a pair of grid stoppers to my PI seemed to completely solve the problem.