Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Blackface5f6 on April 11, 2021, 07:28:25 am
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Hello everybody,
I bought a VibroVerb AB763 clone board, chassis and transformers and converted to the better chassis from --- but now I have a short circuit. Don't worry, I'm using Limiter. ; D
I can also see the arches in GZ34 :(. I tried to localize. I checked the circuit hundreds of times. Everything is actually fine.
The short circuit is somewhere in the output stage. I did the following to locate it, but there is still a short:
1. Separation of grid resistors with pin 5 of the two 6L6GC
2. Separation of the heater from the power tubes with the remaining tubes (preamp, phase converter, reverb, vibrato). The rest was practically simply unconnected.
3. Separation of the choke with the rest of the circuit such as phase inverter, preamp, vibrato and reverb
so it is practically only the mains transformer, tubes rectifier GZ34, power amplifier 2x6L6GC, choke and output transformer.
The short circuit is gone when I do the following:
1. Separation of the power supply from the choke with the CT of the primary of the transformer, so the transformer has no power supply
or on primar side.
2. Separation of the power supply from the choke with two 470 ohms to the grid pin 4 of the two 6L6GC
Could I assume that the short circuit is in the output transformer?
LG
Agu
PS: I used the schematic and layout from TAD
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I would rule out the filter caps before blaming the OT.
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I would rule out the filter caps before blaming the OT.
Thanks Sluckey. I pulled out the Elkos. on D Point and C Point and there is no more short circuit. The Elkos are new. Somehow there is a mistake on the Elkos Circuit. Although the circuit is very simple :w2:
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Reconnect the cap to point D but leave the point C cap disconnected. Still no short?
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I have reconnected the D and there no short
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Then the problem must be the node C cap. Replace it.
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without connecting C but connect the secondary OT to speaker. I get short again :w2:
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without connecting C but connect the secondary OT to speaker. I get short again :w2:
Really! You never mentioned the OT secondary. Leave the OT secondary disconnected and connect the node C cap. Is there no short?
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Sorry for not to mentioning.
I pull out the black cable from OT to Ground Jack Speaker and I still get short
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So I pull out all OT secondary from speaker jack and without C but I get short :w2: :w2: That makes me totally confused
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Post some big hi-rez pics.
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I have pulled out the D and C but reconnect the Secondary OT to Speaker Jack. The short is gone
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The new Elkos circut but has also same short.
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I have pulled out the D and C but reconnect the Secondary OT to Speaker Jack. The short is gone
As I measure the D node to Ground, it is abot 4,9 Ohm :w2: so that means the Elkos to D node is fault, doesn't it?
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I don't know. You keep telling me something different every time you post.
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Sorry Sluckey, it is because I am trying to track the mistake....okay that's fine I will try to find and I will post the result later
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I have pulled out the D and C but reconnect the Secondary OT to Speaker Jack. The short is gone
As I measure the D node to Ground, it is abot 4,9 Ohm :w2: so that means the Elkos to D node is fault, doesn't it?
Why do you say that? You have the D caps disconnected.
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I have pulled out the D and C but reconnect the Secondary OT to Speaker Jack. The short is gone
As I measure the D node to Ground, it is abot 4,9 Ohm :w2: so that means the Elkos to D node is fault, doesn't it?
Why do you say that? You have the D caps disconnected.
Yes, I have disconnected and measured positiv lug of the Elko on D and - /Ground lug, I got 4,9 Ohm, so it is short. Now I change the Elko on D and connect all wiring to the 2x6L6 and Speaker, there is no short circuit.
Your 1st sugestion to check Elkos is correct. Thanks a lot Sluckey, you help me to stop speding on new PT.
I will need to connect the heater cables of all 12ax7 but I need to buy new cable. Since, of all my stock is gone. :icon_biggrin:
Once I get final result, sound I post it here.
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As I look at your filter section, I'm seeing lots of potential problems. It looks like there is solder all over the place -- see the left most arrow, is it possible this solder blob is shorting one of your resistors to the chassis?
Are your filter caps wired correctly? It appears they are all shorted together (rightmost three arrows) -- though maybe this is an optical illusion due to the photography.
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I have some more questions but it is nothing to do with the short circuit.
On the 1st filter circuit:
- 2 x Elkos parallel with the 220K Ohm, how the filter works?
- why we need parallel resistor to the elko?
- Can we just replace 140 µF with 440K Ohm in parallel? (Sorry, if it sounds a stupid question from me but I really do not understand)
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As I look at your filter section, I'm seeing lots of potential problems. It looks like there is solder all over the place -- see the left most arrow, is it possible this solder blob is shorting one of your resistors to the chassis?
Are your filter caps wired correctly? It appears they are all shorted together (rightmost three arrows) -- though maybe this is an optical illusion due to the photography.
You are absolutely correct. Thank you Sluckey.
As I told just for testing, I have replaced the 1st capasitor (attached the pic).
I will buy new board for the capasitor circuit. Since, it was not well done. I will also change the main board, just to make sure to avoid any short circuit on the main board as well.
Modify message
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I have some more questions but it is nothing to do with the short circuit.
On the 1st filter circuit:
- 2 x Elkos parallel with the 220K Ohm, how the filter works?
- why we need parallel resistor to the elko?
- Can we just replace 140 µF with 440K Ohm in parallel? (Sorry, if it sounds a stupid question from me but I really do not understand)
They are not parallel. They are series connected. There should be one 220K resistor across each cap. This resistor insures the voltage will split equally across each cap. Two caps are connected in series to obtain a higher voltage rating.
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Okay, that for the explaination. Therefore the voltage rate of that elkos are 350 Volt. Although they are the 1st stage. So, actually the rate will be 700 volt (2 x 350 Volt in series). Thank you Sluckey :notworthy:
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Finally, I can back to fix the short issue. It was because of broken Elkos. I also changed all Elkos in power suplply section.
But the amp have now problem with Reverb. There is a lot of noise I think it comes from Reverb:
1. When I turn on standby switch there is noise about 5 seconds, it sounds like noise from tremolo
2. After 5 seconds the "tremolo" noise is gone but there is very strong "reverb" noise
3. When I push the Reverb to maximum the "tremolo" noise comes again
4. the reverb is also very weak. It isn't like VibroVerb should like ... :w2:
The Normal Channel sounds very nice. Both (Normal and Reverb/Tremolo) channel are also very loud :icon_biggrin:
I will also try to record later and will be posting the sound(noise) sample here.
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Here is the "reverb" noise. When I turn the Reverb to 10 the noise becomes more hearable.
I have added even 470 KOhm between 500pF and Pin 7 of 12AT7 (Reverb Driver) but the noise is still there.
I have attached the video as zip file.
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WinZip Pro 10 will not open that zip file.
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Hi sluckey,
attached as winzip file. hopefully you can open it.
Agu
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It's now a valid zip file. I got video but the audio is so weak I can't hear the amp. All I heard was a couple syllables of your voice and that was very weak audio too.
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I have recorded three more video with amp inside of the cabinet. The reverb noise doesn't come any more when I push reverb pot to 10 but the first 5 second niose when turn on standby and tremolo noise is stil there.
Hopefully, you can hear now the sound.
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So, now my vibroverb sounds very nice:
1. put the drill cables to grids of of both 6LG tubes in other way round
2. the grids of V1 were also were also reversed
3. replaced the GZ34. New tube is always good :-)
One thing I want to make it better, the reverb should be earlier comming. Currently, it is effective after 6.5 Reverb Pot. I read from Robrobinette to adjust reverb level: https://robrobinette.com/AB763_Modifications.htm#Max_Reverb_Boost_Mod
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One thing I want to make it better, the reverb should be earlier comming. Currently, it is effective after 6.5 Reverb Pot. I read from Robrobinette to adjust reverb level: https://robrobinette.com/AB763_Modifications.htm#Max_Reverb_Boost_Mod
If you followed the TAD schematic/layout, you used an audio taper 100K pot for REVERB. Replace that with a linear 100K just like all the original AB763 amps use and the reverb will be very strong on 2 or 3.
Don't throw away that other pot though because I bet you'll soon want to put it back in. One of the most common complaints about the AB763 reverb is that it comes on too fast. It's been a common mod to replace the stock linear pot with an audio taper pot. TAD has already done that for you.
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One thing I want to make it better, the reverb should be earlier comming. Currently, it is effective after 6.5 Reverb Pot. I read from Robrobinette to adjust reverb level: https://robrobinette.com/AB763_Modifications.htm#Max_Reverb_Boost_Mod
If you followed the TAD schematic/layout, you used an audio taper 100K pot for REVERB. Replace that with a linear 100K just like all the original AB763 amps use and the reverb will be very strong on 2 or 3.
Don't throw away that other pot though because I bet you'll soon want to put it back in. One of the most common complaints about the AB763 reverb is that it comes on too fast. It's been a common mod to replace the stock linear pot with an audio taper pot. TAD has already done that for you.
Currently, I use Alpha Pot. How about if I use 100k audio taper from CTS?
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Currently, I use Alpha Pot. How about if I use 100k audio taper from CTS?
***NO***
How about you use an Alpha linear pot? Or a CTS linear pot? Or a linear pot of any other brand?
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How about you use an Alpha linear pot? Or a CTS linear pot? Or a linear pot of any other brand?
Thanks Sluckey I will try with a good 100KOhm Linear