Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Toxophilite on January 15, 2015, 01:18:54 am
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Hello
I've finished building a Fender 6G15 Reverb unit inside the Garnet15R chassis that I had
I used a Grundig SE EL84 output transformer that's about a 47-1 turns ratio and from doing the math would seem to be about 5 ohms output with the 6K6Gt (which I have)
I'm also using a 5 henry choke(that's what I had)The tank I'm using measures (with a multimeter) 2.4 ohm in and 173 ohm out
So!
I'm not getting ANY reverb
The unit powers up, there's no smoke, it passes signal into an amp and there's no excessive background noise or hum, but no reverb! when you turn the mix pot all the way to one side the dry signal disappears
I went over the schematic from input to output a couple times all seems to be in order
Checked for bad solder joints, they seem okay
One thing I'm wondering about is the inputs and outputs are grounded to the chassis ala fender amps Is this wrong??
Any suggestions would be much appreciated
THanks!
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HELP!
:help:
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Do all tubes light up? Look for two filaments in the 12A?? tubes.
Post a complete set of tube voltages, ie, all grids (even if zero), cathodes, plates, screens.
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Hi, and Thanks!
All the tubes are lit up(All the heaters are on) and I've swapped in another for each one ( I even swapped in a 6V6 for the 6K6 just in case my 6K6s were duds )
My voltages are a little low as I was tweaking my voltages as initially they were pretty high
I put a 2.2k resistor between the choke and the 1st B+ line to the reverb tranny and 6K6 plate
It wasn't making reverb before I did that either
I'll probably switch that to a 1+ K resistor
The one that seems particularly out of whack is the plate voltage of the 6K6 which should be around 225 rather than 271
(out of curiousity how do you measure heater voltage again..I used to know but it's been a while)
12AT7
1 - 101
2 - keeps changing, MV
3- 1.96
6 - 108
7 - .3mv
8. 1.67
12AX7
1 - 231
2 - 103
3 - 114
6 - 147
7 - .5mv
8. 1.2
6K6GT
3 - 271
4 - 274
5 2.4 mv
8 - 21.63
Edit; Fixed pin numbers. Brad
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On 12 _ _ 7's pins 4, 5 and 9 are heaters. Pins 1 and 6 should have highest dcv's on those tubes.
Look at your voltages, should only be 6.3acv measured from pins 4/5 (tied together) and pin 9.
Brad :icon_biggrin:
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Your voltages look OK. Divide and conquer. The reverb unit makes this easy.
Testing the recovery circuit... Disconnect the plug from the tank output. Touch your finger to the tip of the plug and you should hear a buzz in the amp speaker. The buzz will be similar to the buzz you get from touching the tip of a guitar cable that's plugged into the amp but not connected to a guitar. The mix pot should be turned to all reverb. Got buzz? If no buzz, you have a problem in the recovery stage (1/2 of 12AX7). Not much circuitry.
Testing the driver circuit... The driver circuit is just a low power single ended amplifier. Disconnect the plug from the tank input. Connect an 8Ω speaker to the dangling cable. You should be able to hear your guitar thru the test speaker. Don't connect the output of the 6G15 to an amp for this test. Got sound? If not troubleshoot 12AT7 and 6K6 circuit.
If you get a buzz in the first test and can hear a guitar thru the test speaker in the second test, the circuitry is probably OK and you have a bad tank, bad cable(s) or have the cables reversed.
Double check, triple check all your wiring. Maybe print the schematic and use a highlighter as you trace thru the circuit. Mark each component and wire as you proceed. Verify all component values as you go.
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THanks Brad
i know whach pins are the heaters
and all the heaters on the tubes are lighting up and the tubes are getting nice and warm...but when I put my meter across them I get -2.2mv or something similarly confusing. And that's right from the source as well.
And i just saw your new post S. Luckey and I'll check those things thanks
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buzz on the tank output, but very minimal distorted sounding buzz . A tiny fraction of what I get when I do the same thing with a cable into the amp at the same volume.
No discernable sound out of a speaker hooked up to the input cable (from the transformer)
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12AT7
1 - 101
2 - keeps changing, MV
3- 1.96
5 - 108 <------
6 - .3mv
7. 1.67
12AX7
1 - 231
2 - 103
3 - 114
5 - 147 <-------
6 - .5mv
7. 1.2
Maybe you mislabeled the pin #'s?
Brad :icon_biggrin:
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Good point! a electronic typo :BangHead: . make 5 =6 6=7 7=8 I was thinking an octal socket
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Nothing like self help!!
After several false starts
I used S. Lucky's method and starting at the .01 cap after the plate of the 1st gain stage I tried running things into my tube guitar pre unit I built out of a Akai reel to reel pre/amp. That stage worked but the plate of the plate of the 2nd gain stage wasn't producing any signal. Traced it back to the dwell pot, bypassed it and I'm getting signal through the speaker(hooked up to the reverb transformer as a test unit). Still no reverb and still no buzz off the line coming from the tank. Soo I started tracing it back from the output and eventually discovered the mix pot was bad too.
Both pots were I believe almost brand new but seem to be either for a particular purpose or shot
They only work from the wiper to the ground side and turned OFF at full spin the other direction. They are always open from the input to the wiper...weird!!. Replaced them and now reverb
Good to know my circuit was well excecuted as I was pretty methodical about it all and had checked it at least 3 times..
Thanks for now..(I might be back muhahaha)
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All right! Good job. How does it sound compared the Garnet?
You can try double checking a build with a high lighter next time, if you don't already already, it works great;
http://el34world.com/Forum/index.php?topic=17701.msg178630#msg178630 (http://el34world.com/Forum/index.php?topic=17701.msg178630#msg178630)
Brad :icon_biggrin:
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That is a good suggestion and I will incorporate it into my troubleshooting from now on
I had gone through the entire build methodically starting at the input section by section and all the components were the right values, the joints were good, and nobody was touching anybody they shouldn't etc..It was just two weird and or screwed up pots. I'm pretty sure they're brand new and I paid a pretty penny for them. Luckily I had a couple other ones (one new, one used) .
It has waay more reverb potential than the Garnet..Hilarious amounts in fact. better dry through sound too
It even kicks my super reverb's behind as far as sheer amount of reverb, The tone control is nice too.
It's nice to be driving the long Gibbs tank I had from the Hammond organ. I might try it with this shorter 3 spring tank I have or mount two tanks in the chassis giving more sonic options for recording.
This opens up a smaller stereo live rig for me as I have a few songs that rely heavily on lots of reverb in sections and my only option was the super reverb. The super is an excellent and great sounding amp but in some venues I can't turn it up past 2.5 , it weighs 85lb and is not a 'small' amp.
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The rough reverb beastie sitting upside down atop the plywood box I'm going to mount it in(it will be sanded further, varnished and have nice vintage grill cloth on the front , a handle and feet, no tolex ) It looks larger than the box due to perspective but fits in it easily with lots of room for the tank
I'm probably going to cover up a few chassis holes, maybe build a little aluminun box around the OT and likely add some RCA connectors with better insulated RCA coax between the unit and the tank.
Any other helpful suggestions would be appreciated
All in (not counting time, which unfortunately included many extra hours of troubleshooting) It cost me about negative $30 as I purchased it with something else that I was able to sell for more.
Thanks very much for the help!
For chuckles I'm including a picture of my valuable source of RE-150 Space Echo high fidelity replacement tape
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I have a question
I left the power switch,fuse area and pilot light as it was originally with the Granet
However it seems to come on pretty harshly with a bit of a thunk, smae for shutting off
Any suggestions on taming this? There is a .047 cap in the works by the switch. i thought that was for keeping it from popping.
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And of course another question
The only voltages that are significantly off are the plate and cathode voltages of the 6K6
The schematic says:
grid - 295
Plate - 225
Cathode - 24
I measure (now i swapped in a different 6K6
Grid 292
Plate - 292
Cathode - 17.5
Could the different plate and cathode values be a result of the OT being a little different
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Your voltages are right on.
The only voltages that are significantly off are the plate and cathode voltages of the 6K6
The schematic says:
grid - 295
Plate - 225
Cathode - 24
You're looking at a blurry schematic. The original plate voltage is 285. Look at this clearer schematic...
http://el34world.com/charts/Schematics/files/fender/Fender_REVERB_6G15.pdf (http://el34world.com/charts/Schematics/files/fender/Fender_REVERB_6G15.pdf)
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Cool, and thanks
The low Cathode voltage isn't a problem??
Is that the bias?
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Cool, and thanks
The low Cathode voltage isn't a problem??
Is that the bias?
Yes, that's the bias. Your OT can be a factor in the different cathode voltages. Different tubes, even of the same type, will have more effect on that voltage. 17V is fine and means your tube is running a bit cooler than the one in the original schematic. You got PLENTY of verb, right. I'd say well done. Finished. Now go play Pipeline 100 times! :wink:
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:l2: Bahaha. Just finishing up the box
I'll get to work on Pipeline 93, 94, 95....
\
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Some pictures of the mostly completed unit (some aesthetic work still to be done on the box)
Lastly a picture of the stack all made of recycled repurposed materials and a few inexpensive used purchases
(the biggest of which was the JBL D130 for $75)
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Looks real nice!!!!
(What happened to the front panel fuse?)
Brad :icon_biggrin:
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THat's a good question. There are Two fuses inside the chassis. It came that way so I left the whole,power in,fuses, pilot light, switch and diode bridge as it was (though I added a 3 prong plug) The pilot light is a little smashed too. i might eventually install a more conventional fuse holder where it says 'fuse' and put in a non smashed pilot light too. THough if the unit is blowing a fuse,It would probably be best to take it apart anyway!!
Also the little hole in the front will help cull over curious small children..... :icon_biggrin:
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There are Two fuses inside the chassis. It came that way so I left......:
Ok, just wondering. (Figured you had it inside.)
Also the little hole in the front will help cull over curious small children..... :icon_biggrin:
Probably be a good thing.
Brad :icon_biggrin: