Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: SleepLess on August 30, 2013, 04:02:47 am
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Hi.
I have finished a 5E9A build and I'm worried with the biasing. The amp works ok but is really clean and the tremolo is weak. I have to set it on 10/12 in order to hear it well. So I have calculated my bias and here's what I have:
B+: 375V (232V wall voltage, I'm in the EU)
Cathode resistor: 250ohms
Cathode voltage: 22V
375-22= 353V
Cathode current: 22/250= 0.088 amps = 88mA = 44mA for each tube
Hence 0.044 X 353= 15.532W
A 6V6 max dissipation is 14w so I'm above the 100% max dissipation. The usual recommendation for tubes is 70% which would be 9.8w. In order to get that I should replace the 250 ohms resistor by a 400 ohms which would make:
22/400= 0.055 = 55mA = 27.5mA each tube
Hence 0.055 X 353 = 9.7w
Would you agree with my conclusions? Is there something I'm missing with cathode biased amps? The thing is that amps that are biased too hot generally produce distortion and I'm actually getting the opposite, the amp is too clean for a tweed...
Thanks a lot!
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With a cathode biased output stage, you can bias the tubes to idle at 100% and it won't do them any harm. If you're worried about the idle dissipation, chuck a pair of JJ6V6S in there (they're 14W tubes).
The trem wiggle in a 5E9A drives the 1st stage of the paraphase inverter. The gain of the LFO stage affects the strength of the wiggle, and in the 5E9A, this is moderated by the 10M feedback resistor. Double check that this is connected properly.
(http://www.freeinfosociety.com/electronics/schematics/audio/pictures/fender5e9a.gif)
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Class A need to be a 90% , 70% too cold
http://www.webervst.com/tubes1/calcbias.htm (http://www.webervst.com/tubes1/calcbias.htm)
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I have finished a 5E9A build
IMHO, an absolute winner!!! This amp sounds fantastic! Best sounding trem I have ever heard.
tremolo is weak
Replace your 12AX7. I have built some of these and higher gain tubes work better on this position.
If you are worried about your bias, try 680R on each tube cathode, so that you will know for sure how much current is flowing thru each tube.
Hope this helps,
Best Regards.
R.
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Thanks a lot guys! Once again this forum turns out amazing. What a well of knowledge it is!
I'm gonna buy a 300ohms 5W resistor in order to bring down the bias to 90% (it's more like 110% now with more than 15w dissipation). I'll report back then and post a few pics of my build.
Take care! :worthy1:
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Cathode current: 22/250= 0.088 amps = 88mA = 44mA for each tube
Hence 0.044 X 353= 15.532W
A 6V6 max dissipation is 14w so I'm above the 100% max dissipation. ...
Cathode current does not equal plate current; cathode current is plate current + screen current.
Your amp does not have screen resistors, so there's no easy way to determine screen current (other than the classic method of unsoldering the wire to the screen and inserting a series ammeter).
If it were my amp and I did not see the plates glowing red, I'd leave the amp as it is.
... The thing is that amps that are biased too hot generally produce distortion and I'm actually getting the opposite, the amp is too clean for a tweed...
That's a new complaint for these amps, as people usually aren't happy with the lack of headroom in a Deluxe or Tremolux. At what volume setting does your start distorting?
I used to own a 1955 5E9, and while it would certainly distort it had a range of clean sound available, too.
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The 5G9 is a cleaner sounding amp (but then its got a LTP inverter and fixed bias 6V6s). It also has nicer tremolo.
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If it were my amp and I did not see the plates glowing red, I'd leave the amp as it is.
... The thing is that amps that are biased too hot generally produce distortion and I'm actually getting the opposite, the amp is too clean for a tweed...
That's a new complaint for these amps, as people usually aren't happy with the lack of headroom in a Deluxe or Tremolux. At what volume setting does your start distorting?
I used to own a 1955 5E9, and while it would certainly distort it had a range of clean sound available, too.
I'd say it distorts around 9 (with a strat). Below that it is pretty clean. Don't get me wrong, it's the 15th tweed I've built from the 5F1 to the 5F6A and all models in between. This amp distorts way after the others. Even the Bassman was crunchier on 6... I don't get it.
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I'd say it distorts around 9 (with a strat). ... This amp distorts way after the others. Even the Bassman was crunchier on 6... I don't get it.
Have any of your other tweeds been a 5E3 or similar? Does this amp distort at a similar room volume as the 5E3?
This is not volume knob setting, but similar loudness.
If yes, did you use a different brand/type of pot in this build? Audio tapers vary; the general range is 10%-30% taper. A true logarithmic curve will have about 10% of the pot's total resistance at 1/2 rotation. A 30% taper pot will have 30% of the pot's total resistance at the same point.
WeberVST does (or did) sell pots with a 30% taper. Because these are higher resistance for the same volume position (except 0 and full-up), they seem louder/more distorted for the same volume knob setting. If you switched to 10% taper pots, the same amp would seem like it stayed cleaner, longer, even though the amp is doing exactly what it always did with a given volume pot resistance.
The amp works ok but is really clean and the tremolo is weak.
Does your oscillator show the same/similar voltages as noted on the 5E9A layout? That will give you an idea if the trem circuit is creating a strong enough signal. Your 6V6 cathode voltage is actually a bit lower than on the schematic, so if anything that should result in a bit more trem strength and a dirtier sound.