Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mayburyds on March 27, 2014, 03:24:32 pm
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Hi,
Recently built a 5E3 with much help from the people here. But although it's always sounded good it never really quite hit the 5E3 spot so to speak. Bit too bright and not a lot of sag.
Anyway, in the Tube Doctor circuit diagram it shows the 230v tap showed be connected and the 240v tap not used. And although the UK is supposed to be at 230v the reality is it's still 240v.
I checked the voltage supply coming out the wall and it's over 240v, so I swapped to the 240v tap and bingo, much improved tone. Not quite as loud as it was before but still loud enough. More drive, breaks up sooner and not as bright. All good.
I also have a UK Blues Deluxe Reissue and that is set for 230v. So I thought I would change that to 240v aswell.
Now hang in here, awhile back this amp decided to start smoking despite all the government warnings! Turned it off and left it for a bit, turned it back on and all seemed well.
Back to the voltage change. Having changed to the 240v tap I thought I'd check the bias at the test point and it read 0v! The voltage at the variable resistor controlling the bias voltage is -55v at it's highest point. Put it back to 230v and got the same readings. Obviously this must be related to the smoking incident!
All the valves are glowing but the volume is low i.e. I can wind the volume up half way without my ears being ripped off.
Any ideas what the problem might be with 0v at the bias point and is it ok to switch to the 240v taps?
Cheers.
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Now hang in here, awhile back this amp decided to start smoking despite all the government warnings! Turned it off and left it for a bit, turned it back on and all seemed well.
All is not well.
Any time any amp starts smoking yes turn it off but DON'T turn it back on until you find out and fix why it was smoking.
Pull the power tubes on the amp and DON'T put them back in until you absolutely make sure the -bias is working.
A 5E3 is cathode bias, the Blues Deluxe RI is grid bias. Changing the PT primary tap on the BD RI will change the -bias. When you did that you should have checked and if necessary adjusted the -bias.
Brad :w2:
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... I also have a UK Blues Deluxe Reissue and that is set for 230v. So I thought I would change that to 240v aswell.
Now hang in here, awhile back this amp decided to start smoking despite all the government warnings! Turned it off and left it for a bit, turned it back on and all seemed well.
Back to the voltage change. ...
Do you write Tarantino movies? :l2:
Just kidding... I want to make sure I understand in what order things happened. I gather the Blues Deluxe started smoking, but before you switched the amp to run on 240v? Is that correct?
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Brad,
OK, will pull the tubes.
I was trying to check the bias after changing the tap, that's when I found out it was 0v.
Trouble I have now is I have no idea where to start looking, so I'll have to take it a tech.
Cheers.
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The BDRI was smoking a while back but then I started using the 5E3.
It was only today that I switched it on, no smoke by the way, to change the 240v tap.
Cheers.
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Any ideas what the problem might be with 0v at the bias point
Unknown. Since all the supply voltages in the amp will have decreased when you switched to the 240V tap, I see no reason why anything would smoke. Maybe the tap-swapping is a coincidence.
is it ok to switch to the 240v taps?
Of course; that's what you're supposed to do. (Driving a transformer with too much voltage causes it to saturate)
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Ok, nobody spotted the deliberate mistake then!
mV not V :BangHead:
Set the bias to 60mV, all seems good.
Thanks for the input and advice, sorry for wasting peoples time over the calibration mistake.
Cheers.
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Hi,
Thanks for the info.
If I understand it correctly you plug this box into the mains and then plug the amp into the box and it reduces the mains down 6-12 volts? Could be useful.
When I changed the voltage on my 5E3 to 240v the tone was much improved, but this last couple of days the tone has a spiky edge to it. Wondering if these changes in tone could be caused by fluctuations in the house power supply.
It would be great to have a box that guaranteed a 240v supply.
Cheers.
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Get a Variac if you wish to control your wall voltage. You can lower them to achieve what you want. Mine will increase the voltage as well. I take one with me for gigs and use one at home that is inline with my amp power strips. The voltage at my home is consistent at 126vac except in summer then it pulls down in the afternoon, but my old amps I have are designed closer to 117vac. Makes a big difference and I also feel better about not overpowering the old amps I have.
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Thanks, I'll have to have a look to see what they are all about. Cheers.
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Sorry, bit late with reply.
Yep 0v is the same is 0mv. Problem was that I had the meter on v not mv. So the mv reading was so small it didn't show up. Switching to mv and bingo a reading.
Cheers.
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If bias is at 0V assuming no error, wouldn't this be 0mv also? 0.000 v is same as 0 mv? (I don't do metric well).
Not to a meter.
Say the lowest Volts range is 2v. Say the measured bias is in fact 60mV, but measured on the 2v range.
The Wavetek 27XT meter (a typical inexpensive service-grade meter) has selectable ranges for voltage measurements. Accuracy for all ranges is specified as +/-0.5% + 1 digit. 60mV is 3% of full-scale and should be resolvable; however if the user has the meter on the 20v range, the voltage is 0.3% of full-scale and probably lost in noise. If set to the 20v range or higher, the meter will almost certainly say "0v".
The Hewlett-Packard 412A (a very good d.c.-only VTVM) has minor divsions (smallest scale marking) of 50mV on the 3v scale. Hopefully you would notice the meter deflecting a hair off of 0v, but you might miss it. On the 10v range, the minor divisions are at 100mV; again you might miss that. Set to a higher range, this meter would show no obvious indication other than "0v".
So 0v = 0.000mV, but 60mV could look like 0v depending on the meter range.