Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Ed_Chambley on December 22, 2011, 09:15:07 pm
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Just started and have a problem. Maybe someone could give me some input.
Numbers
v1
1-157
2-0
3-2
6-215
7-0
8-3.4
v2
1-261
2-0
3-1.54
6-460
7-261
8-263
v3
1-323
2-65
3-111
6-315
7-70
8-111
v-4 and 5
3-460
5,6--59
All heaters 6+acv, Power Supply reds 340, Yellows 5v
Any help pointing in the right direction would be appreciated.
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Screen-grids on 6L6es?
-59V seems excessive; but if you lean on it you should still get BIG output.
Did you add the 1 ohm cathode resistors usually used today? What is 6L6 current?
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I jumped the gun a little. Did not have pin 4 of v4 and 5 connected to the 470ohm resistor. I have a very low bias, but I would have thought the bias dropping resistor would be close, that is why I have not changed it yet. What I have now is big output, with a loud hum.
I did use the small cathode resistors. I'll be searching out the hum. Any place to start?
If I do need to adjust the bias resistor, it is 1k now. Going down in value will raise the bias or is the reverse true.
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There is something wrong with your bias on V1 triode2... pin 8. That is a bit too high, I would investigate.
V2, pins 6,7,8, all too high. Even for a high voltage (460 not 435V B+) those are a bit too high. I would look
at if your B+ dropping resistor is correct, or if the tube is pulling current..
OTOH, I see your PI tube is a bit high too... Is that 4.7K resistor after the choke 4.7k or something else?
Hrmm.. those voltage may be "OK" in the respect that your B+ is 460V. I would prefer to get it down a bit
more to give you that Fender 5F6A tone... on the scehmatic the voltage after the 4.7k should be around
385V... http://www.el34world.com/charts/Schematics/files/fender/BASSMAN_5F6A.pdf (http://www.el34world.com/charts/Schematics/files/fender/BASSMAN_5F6A.pdf)
As PRR noted, I would check my 6L6 current and bias voltage.
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Ok thanks. Got the B+ from 460 to 350 after the 10 k dropping resistor. Hum gone. I only have -8.2 bias, but I have a set of old tubes in it to fire it up. I will change tubes and get the bias in line. Hum is gone, but both volume controls have an effect and I have no jumper in place. Is this normal or do I have something funky in the wiring? Not a large effect like they were jumped, but slight.
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I only have -8.2 bias, but I have a set of old tubes in it to fire it up. I will change tubes and get the bias in line.
No! Pull the output tubes and don't put them back in until you have the correct bias voltage on pin 5 of each empty socket. Nothing else matters until this is right.
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I only have -8.2 bias, but I have a set of old tubes in it to fire it up. I will change tubes and get the bias in line.
No! Pull the output tubes and don't put them back in until you have the correct bias voltage on pin 5 of each empty socket. Nothing else matters until this is right.
What am I looking for, about -46?
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I only have -8.2 bias, but I have a set of old tubes in it to fire it up. I will change tubes and get the bias in line.
No! Pull the output tubes and don't put them back in until you have the correct bias voltage on pin 5 of each empty socket. Nothing else matters until this is right.
What am I looking for, about -46?
Roughly, what I normally do is shoot for something around there, then with the tubes in and a meter across the 1ohm
bias resistor on the output tubes, fire it up. If it starts climbing over 0.040VDC really quickly, shut it down, and play with
your bias range resistor (or crank your bias pot the other way then try again).
Personally, I shoot for 35ma, but it depends on B+ in this amp. Also, and few people do this, check your bias, get them both
to around 30-35mA (0.030 to 0.035VDC across the one ohms), then WAIT 30 mins with it on, then recheck. I do this at 30 mins
then at one hour, but in general, it will change from 0-30mins. If you want to be really nuts, do this, then at about 100 hours
of use, check the bias again. After that you will be good until they need to be replaced.
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check this out for your biasing.
http://www.diyguitarist.com/GuitarAmps/PowerTubeBias.htm (http://www.diyguitarist.com/GuitarAmps/PowerTubeBias.htm)
Ernie
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35ma is loafing for a pair of 6L6GC's.They can easily take 45-50ma at 430v
To me 35ma on a Bassman sounds sterile.
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I only have -8.2 bias, but I have a set of old tubes in it to fire it up. I will change tubes and get the bias in line.
No! Pull the output tubes and don't put them back in until you have the correct bias voltage on pin 5 of each empty socket. Nothing else matters until this is right.
Bias dropping resistor ended up being 6k8. I had -45 on pin 5. Installed output tubes and plugged into the probes I have on back. I have a sweep of -30 to -46 with the tubes I have in there. They are not the tubes I plan to use, but one of my Tung-sol 5881 is dying. Got a friend send me some NOS Tug-sol 5881. I will have them in a couple of days. I do have a matched set of GE Clear 6l6GB, some 6l6GC Matched Winged C's and some matched JJ6L6GC's. Until I get the Tung-sols, what would you stab in? As so far as preamp, I have everything. V-1 Raython 12Ay7, V-2 and 3 RCA Black Plate 12AX7. Probably a good setup for preamp, but would like suggestion on power tubes.
Amp is now dead quiet and all voltages check out.
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I have a sweep of -30 to -46 with the tubes I have in there.
OK. I'm assuming that's a sweep of -30 to -46vdc bias voltage as measured on pin 5? That voltage should not be affected by which tubes you use.
The next measurement that's probably more important to you would be the millivolts measured across each of your 1Ω cathode resistors. This reading is dependant upon the bias voltage setting on pin 5 AND the particular tubes you use. This reading represents the actual current flowing thru the tube and ideally you would want the two tubes to be the same (well matched). But since this is not a hi fi amp, close is good enough.
I would not hesitate to use any of the tubes you mentioned. Sounds like your about done except for playing.
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Ah,you are using NOS 5881's! Indeed 35ma will be fine,not much more than that.That amp should sound pretty good.
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I have a sweep of -30 to -46 with the tubes I have in there.
OK. I'm assuming that's a sweep of -30 to -46vdc bias voltage as measured on pin 5? That voltage should not be affected by which tubes you use.
The next measurement that's probably more important to you would be the millivolts measured across each of your 1Ω cathode resistors. This reading is dependant upon the bias voltage setting on pin 5 AND the particular tubes you use. This reading represents the actual current flowing thru the tube and ideally you would want the two tubes to be the same (well matched). But since this is not a hi fi amp, close is good enough.
I would not hesitate to use any of the tubes you mentioned. Sounds like your about done except for playing.
Now done with playing and I get it. I always wondered why players were so crazy about the bassman. I put in a pair of Winged C's, matched within 1ma at idle. The deal is you don't have to hunt for a good tone, just plug in, turn up to about 7 and play. Move the knobs and it sounds different, but still good. Every note is clean and precise, but hit a hard chord and it rocks. Very nice amp with a very woody sound. It is not as loud as I expected it to be which is a good thing. I ran it through several different cabinet configurations and they all sound good, but the 4/10 with the weber alnicos on top and ceramics on the bottom give the best blusy tone. I've got a 2/12 with a silver bell and a blue dog and it chicken picks wonderfully.
Thanks for the help, I have a very nice keeper.