Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => EZ Board postings => Topic started by: Geezer on June 17, 2005, 08:26:26 pm
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Hoffman Amplifiers
> Tube Amp Building - Tweaks - Repairs
> Add filtering or bridge rectifier??
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tubenit
Junior tube assistant
Posts: 1
(1/19/04 8:37 am)
Add filtering or bridge rectifier??
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Filtering vs. bridge rectifier question on 2nd page.
OT question 2nd pg
Solution on pg 2 of the post
I'm rebuilding a Gibson Scout GA-17RVT. I had to replace the power tran with a Hammond 261G6. 250 volt 130ma. The rectifier tube is getting 250volt (new 6CA4), but the output tubes are 160 volt on plate.It has new filter caps 20uf and I tried several caps of the same and all read the same. I'm new at this. Could it be the cathode resister needs changing 6AQ5 tubes. I get a 8.5 volt drop with a 250 ohm resistor? All tubes are new. Tubenit
Edited by: tubenit at: 2/2/04 6:26 pm
tubenit
Junior tube assistant
Posts: 2
(1/19/04 9:47 am)
added info/question?
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I am also wondering whether the output trans could be weak and that have an effect on the low wattage on the 6AQ5 power tubes? They're reading 160volts and should be closer to 250-300volts as I understand it. tubenit
jc
I only work on Fender's
Posts: 630
(1/19/04 10:25 am)
Re: added info/question?
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I would expect to see voltages in the 340-350 range with a CT 250v transformer.
Edited by: jc at: 1/19/04 10:26 am
jc
I only work on Fender's
Posts: 631
(1/19/04 10:34 am)
Re: added info/question?
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What are your voltages at each of the filter capacitor nodes?
This amplifier has a tremelo that varies the bias through the cathode resistor? Are you sure all of the tremelo circuitry is OK. This amp looks like a cathode biased amp otherwise. Hopefully someone will check my thoughts on this idea but I would disconnect the tremelo circuit and ground your cathode resistor to see if the rest of the amp works.
Edited by: jc at: 1/19/04 10:37 am
jc
I only work on Fender's
Posts: 632
(1/19/04 10:55 am)
Re: added info/question?
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One other thing. I am not very familiar with this type of rectifier but the schematic looks like the PT would have a 6.3v winding for the rectifier and another for the heaters. The Hammond that you are using looks like it only has 1 6.3 V winding.
That is just a guess.
Hopefully someone who is more familiar with this type of rectifier will chime in soon...
Edited by: jc at: 1/19/04 11:00 am
ganzonimx
Hey get your own solder
Posts: 335
(1/19/04 11:01 am)
Re: added info/question?
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What voltage do you get on pin 3 of the rectifier with all tube pulled?
Cris
tubenit
Junior tube assistant
Posts: 3
(1/19/04 11:16 am)
response
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I will check the #3 pin on the rectifier tonight and look into the tremelo concern and repost. The filter caps positive end has 160 volts on it as does the output tube plates. There are two green 6.3volt wires, two red 250 volt plate wires and a red/yelow CT wire. All the tubes are getting the 6.3 volts. Thanks and I'll check out the rest. I'm still wondering if a bad/weak O.T. could play a role in this? tubenit
tubenit
Junior tube assistant
Posts: 4
(1/19/04 12:02 pm)
2ndry PT wiring same
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I should add that the hammond PT 2ndry wiring is identical to the original PT and hooks up the same down to the same colors, etc. tubenit
ganzonimx
Hey get your own solder
Posts: 338
(1/19/04 4:46 pm)
Re: 2ndry PT wiring same
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If on the caps positive side you get also just 160V, you probably find also 160V on the rectifier pin 3. The OT should not affect the plate voltages.
I think the rectifier does not work correctly. You could try with a different rectifier or replace the rectifier with two 1N4007 diodes. Pull the rectifier and put in two diodes with the cathode (stripe) on pin 3 and the anode on pin 1 resp. pin 7 and check the voltages again.
With diodes you'll get B around 350V. It should not harm the 6AQ5's during a test run, but it might be to high for them without changing the bias resistor.
I think the tremolo circuit drives up and down a grid bias voltage that is added or subtracted to the bias voltage from the cathode. It should not affect the plate voltage.
Cris
tubenit
Junior tube assistant
Posts: 5
(1/19/04 9:09 pm)
Re: 2ndry PT wiring same
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THANKS for the help! I think you nailed the problem with a bad rectifier tube. Ironically, it was new but I replaced the socket after installing it and maybe the old socket messed it up. Q-1)
I will go the tube route, but if I put in a solid state rectifier would the wiring need changing? The 6.3 volt goes to the rectifier and then all other tubes in a series (?) Also the amp will make a crackle when playing the heavy E string on the 13th fret. Q- 2) I am wondering if that vibration & frequency is on the bad rectifier tube or a shorted resistor? Would be nice if the tube takes care of both. Oddly, as is ..the amp has a good sound with nice harmonics and compression as is and the reverb and tremelo work. I've replaced almost all the caps and maybe 10 resistors.
tubenit
Junior tube assistant
Posts: 6
(1/24/04 8:14 am)
HELP - Please?!!
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I am at a loss.I got a Weber CopperCap WCA4 & tried the diode thing - same voltages are being read 160 DC on pin 3. With all tubes pulled, I get 200 DC. With the output transformer and cap disconnected from pin 3, I get about 125 DC there. All wiring to all sockets is same as the original and matches schematic. Pins 1&7 read around 250 plus AC. The Hammond 261-G6 PT is rated 130ma @ 250 volts. Is the new PT bad or is it possible that the OT as a short that somehow lowers the voltage? Any direction or advice would be appreciated. Thanks!
ganzonimx
Hey get your own solder
Posts: 345
(1/24/04 11:19 am)
Re: HELP - Please?!!
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I suppose you did hook up the PT correctly:
PT primaries (black) to the power inlet.
The two HV secondaries to pin 1 and 7 of the rectifier socket, (they are probably red wires)
HV center tap to ground/chassis, (the C.T. has a other color then the HV secondaries)
Heaters (probably green wires), to pin 4 & 5 of the rectifier socket and to all other heaters.
If you disconnect the wire from pin 3 of the rectifier that goes to the board (with the rectifier or copper cap in place) you should measure between pin 3 and ground the expected voltage of around 300V.
If this is not the case, I think the PT is funky...
Cris
tubenit
Junior tube assistant
Posts: 6
(1/24/04 2:20 pm)
Re: HELP - Please?!!
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The secondary wires are as you say which matches schematic and original wiring. Red wires on pin 1 & 7 (reading 250 volts AC) and pins 4 & 5 (read 6.3 volts) on the rectifier and all other filaments/tubes. red/yellow CT is grounded. The primary wires are hooked up as original also. (I photographed and drew out everything plus had a schematic) before I changed out the PT. The original primary wiring has the AC outlet cord with the white wire on the back of the fuse and the black on the back of the on/off. The amp works including reverb and tremolo. It just isn't loud (less than my VibroChamp) and the voltage on the 6AQ5 plates should be 255volts but still read only 160 volts matching the reading on pin 3.
The place that sold the PT rated it as 12-15 watts however I notced on a website out of London that rated the Hammond 261-G6 as a PT choice for a 3 watt amp?? The thing has 250CT 130ma ratng. I would think it should work?
You think it's defective or the information from the seller is inaccurate and 12-15 watts is way over the true power? Any guesses? I am VERY appreciative of the help!! Thanks!!
jc
I only work on Fender's
Posts: 641
(1/24/04 2:51 pm)
Re: HELP - Please?!!
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From the hammond site
261G6 250V C.T. @ 130 ma 6.3V @ 2A
this is plenty for the amp.
jc
I only work on Fender's
Posts: 642
(1/24/04 2:57 pm)
Re: HELP - Please?!!
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This is really more a question than anything...
On the schematic the heater circuit does not have a center tap and does not have the 2x100 ohmers. I alway thought you had to have one or the other. Is it because this winding feeds the recitifer and the heaters?
Edited by: jc at: 1/24/04 3:00 pm
tubenit
Junior tube assistant
Posts: 7
(1/24/04 3:36 pm)
Re: HELP - Please?!!
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I don't know the answer to that but maybe someone else does? I can tell you the new PT and the rectifier are wired to match the original wiring and the schematic. And you are correct there is not a CT on the 6.3 filament. The only CT on the original was connected to the red/plate wires. The 6.3 volt wires go to pin 4&5 on rectifier and then in a series(?) to all the other tu