• Guys, we've spent considerable money converting the Les Paul Forum to this new XenForo platform, and we have ongoing monthly operating expenses. THE "DONATIONS" TAB IS NOW WORKING, AND WE WOULD APPRECIATE ANY DONATIONS YOU CAN MAKE TO KEEP THE LES PAUL FORUM GOING! Thank you!

Troubleshooting Advice - Friedman Runt 20

dman_vegas

Member
Joined
Apr 5, 2021
Messages
30
I have a Runt 20 combo and just now the clean channel cut out. I changed the tubes a while back and it’s been working fine. I rarely even use the clean channel, but I’d still like to fix it. Maybe a tube went bad?

I have some extra tubes I can swap in, but I’m not sure which one/s are for the clean channel. I can identify the 3 gain stages, but I have no idea really. Any advice is appreciated.
 

PaulD

Active member
Joined
Jun 25, 2007
Messages
590
I'm not familiar with the amp but looking at the website it has 3 x 12AX7 preamp valves (one of which will likely be the phase inverter) and 2 x EL84 power amp valves. The fact that one channel is working rules out any problem with the EL84 power amp valves so swapping the 12AX7's out should confirm if it is a valve problem or not. If you have a known good 12AX7 try swapping it with each of the valves in turn to see if it solves the problem.
 

Wally

Well-known member
Joined
Feb 27, 2003
Messages
3,357
High dollar amp. Got a tech? I don’t find a schematic, but with only three 12A-7s there are not very many ways to get Two channels out of it. The 12A-7 closest to the input Jack, V1, may contain the input stages for both channels…one triode per channel. One triode of the next tube..V2..is likely the second gain stage of the overdrive channel with both channels being summed and passed through the second triode of V2??? This is just a WAG. This would give one channel one more gain stage than the other. Or…there may be other tricks to achieve more gain in that gain channel. A good tech could wuss it out with the amp on the bench in front of them.
At any rate, a known good tube in V1 would be the first and simplest thing to try.
 

dman_vegas

Member
Joined
Apr 5, 2021
Messages
30
So I take the thing apart (i have the 20W combo) and figure I might as well try it out one more time before changing any tubes. The clean channel was working! I put it back together and guess what... the clean channel is not working. I inspected all the tubes and they looked ok (for what that's worth). Now I'm mad...
 

Wally

Well-known member
Joined
Feb 27, 2003
Messages
3,357
How does one switch the channels? Does the Clean channel not work when the footswitch is plugged in but work when the footswitch is not plugged in? If the footswitch indicates a problem, I would be taking it apart…IF I knew what I was doing, right?
i would have that chassis pulled again to see if it goes back to working properly…and note the differences….footswitch? If the only difference is that the chassis is in or out of the cab, then I get a ‘chopstick’…some sort of non-conductive poking stick..l.and I start pushing and tapping connections, tube sockets, switches….especially that channel switch and all wiring to and from it. There is something that has a connection problem….or if there is a relay in that switching circuit, one might suspect that. It is likely to be something simple, though, it seems to me.
 

dman_vegas

Member
Joined
Apr 5, 2021
Messages
30
Thanks for all the advice! I heard from Friedman and the V1 stage is the one and only tube for clean. This time, I just replaced the V1 12AX7 (Tung-Sol to a Chinese ARS JJ/Tesla) and all is good! Maybe that tube didn't like being upside down, lol. Now it's in the garbage can and I can sleep well tonight!
 

akstrat61

Well-known member
Joined
Feb 16, 2004
Messages
1,792
My suggestion to anyone who has tube amps is to buy a tube tester. They can help diagnose problem, identify weak tubes and save a ton of time. MHO
 

dman_vegas

Member
Joined
Apr 5, 2021
Messages
30
My suggestion to anyone who has tube amps is to buy a tube tester. They can help diagnose problem, identify weak tubes and save a ton of time. MHO
That's a good idea. I could have used one for sure. Any good tube testers you can recommend and how much do they cost?
 

Wally

Well-known member
Joined
Feb 27, 2003
Messages
3,357
9
Thanks for all the advice! I heard from Friedman and the V1 stage is the one and only tube for clean. This time, I just replaced the V1 12AX7 (Tung-Sol to a Chinese ARS JJ/Tesla) and all is good! Maybe that tube didn't like being upside down, lol. Now it's in the garbage can and I can sleep well tonight!
I figured as much. And did you inquire of them If the gain stage input stage was the s3cond triode in that tube. It has to be, ime, but I always like to ask someone who knows the circuit…..because there is a small,possibility that the gain is done in the solid state doma8n. (8^0????and one would really like to know that. With the amp 9n a bench, I would have the answer to that…or…do this…pull V1. Does the gain channel still function? If not, then that V1 tubes contains the 8nout gain stage for each channel and V2 MAY be the second gain stage for both channels with the second triode 8n V2 being the added gain for the Gain channel. ??? Reinsert V1 and pull V2. If both channels go dead, the. One might surmise that my thought is how the signal path functions. If only the gain channel goes dead, then you know that the clean channel has only one gain stage. If both channels go dead, then one starts to wonder about the gain in the Gain channel….how is it done.
There are all sorts of reasons for not putting a schematic in the owner’s package with a new amp, right? A good tech will know when they open it up how th8ngs are arranged, but pulling tubes and observing what happens with the signal can tell one quite a bit.
 

dman_vegas

Member
Joined
Apr 5, 2021
Messages
30
9

I figured as much. And did you inquire of them If the gain stage input stage was the s3cond triode in that tube. It has to be, ime, but I always like to ask someone who knows the circuit…..because there is a small,possibility that the gain is done in the solid state doma8n. (8^0????and one would really like to know that. With the amp 9n a bench, I would have the answer to that…or…do this…pull V1. Does the gain channel still function? If not, then that V1 tubes contains the 8nout gain stage for each channel and V2 MAY be the second gain stage for both channels with the second triode 8n V2 being the added gain for the Gain channel. ??? Reinsert V1 and pull V2. If both channels go dead, the. One might surmise that my thought is how the signal path functions. If only the gain channel goes dead, then you know that the clean channel has only one gain stage. If both channels go dead, then one starts to wonder about the gain in the Gain channel….how is it done.
There are all sorts of reasons for not putting a schematic in the owner’s package with a new amp, right? A good tech will know when they open it up how th8ngs are arranged, but pulling tubes and observing what happens with the signal can tell one quite a bit.
I heard from Friedman amps and the response was immediate. I'm impressed that they got back to me so quickly, great service!

I asked what stage was the clean channel and the reply was V1. It worked and then I let them know that I have 3 different 12AX7 tubes - an ARS JJ/Tesla in V1, a Mullard in V2, and a Tung-Sol in V3 and asked for a tube set recommendation.

Below is the response from Dave Friedman.

Only thing the clean channel is is the first v1 tube.. Well the Mullard and the tungsol are not the best choice for their positions.. best would be a sovtek 12ax7lps in v3 phase inverter and maybe move the jj over to v2 and use the tungsol in v1.. Its a issue because there are no chinese tubes right now..
 

corpse

Well-known member
Joined
Jun 9, 2007
Messages
4,306
Just for a WTH ask them or try a 12AU7 in V1. It's pretty cool in any Fender.
 

Wally

Well-known member
Joined
Feb 27, 2003
Messages
3,357
d_man Vegas wrote:

“Below is the response from Dave Friedman.”

“Only thing the clean channel is is the first v1 tube.. Well the Mullard and the tungsol are not the best choice for their positions.. best would be a sovtek 12ax7lps in v3 phase inverter and maybe move the jj over to v2 and use the tungsol in v1.. Its a issue because there are no chinese tubes right now..”
yes, I have to accept that the Clean channels as at least one gain stage in that first tube..V1. however, in order to understand more thoroughly how this amp works when one does not have a schematic, one has to use a bit of systematic analysis.…as I outlined above.
I’ll try again….there are two triodes inside each of those 12A-7s. One of those 12A-7s is occupied functioning as the phase inverter. That leaves four triodes to function in two ‘channels’. Since you lost the Clean channel, that means that the two channels Do NOT share a common input stage triode. This is why the Gain channel continued to function. Pull V1….that Gain channel will NOT function, either, more than likely. Try it….we all are on a learning curve. Who knows…I could be wrong, but if I am then I am going to think that It is very hard to have an all-tube preamp that does one clean and one gain channel. if this experiment shows that the two triodes in V1 function separately as input gain stages for both channels, then do this. Reinsert V1 and assure that both channels are working. Then pull V2. What happens? Again…my guess is that the Clean channel may well continue to function while the gain channel will be dead. If this is so, we will have learned something about your amp. Or…we will have learned something other than what I am guessing is going on. IF both channels cease to function with V2 out, then we might surmise that both channel go through one of V2’s triodes with the sec9nd triode 8n V2 being applied solely to the Gain channel.


with this type of approach then IF sometime in the future the Gain channel ceases to function, you can replace V1 to see if the Gain channel triode has gone out…..as the clean channel triode failed you recently, right? Or….if that does not cure the problem ,you can replace V2 with a known good tube. The channel may come back up. If it does, then you know you had a bad tube there. If these tube substitutions do not return the amp to working status, then the amp has more serious problems in the circuit.
I hope this helps you understand your amp a bit better. I think it may.
fwiw, this is what a tech would do if you dropped it off. Hey, I do this in front of my customers so that IF they are caught in the middle of a situation where the amp needs to work,they might be able to install a tube or two and contNiue working/playing. Imho, D. Friedman could have quickly shared this information with you as an owner of his product. A simple signal path diagram in the manual would reveal these basics without compromising any secrets he thinks he has built into that circuit, and said diagram along with a simple narrative could have aided you in dealing with this problem in about two mI uses time.
Fwiw, there are very few secrets in any of these circuits when the circuit is being looked at by an experienced tech.
 
Last edited:

dman_vegas

Member
Joined
Apr 5, 2021
Messages
30
d_man Vegas wrote:

“Below is the response from Dave Friedman.”


yes, I have to accept that the Clean channels as at least one gain stage in that first tube..V1. however, in order to understand more thoroughly how this amp works when one does not have a schematic, one has to use a bit of systematic analysis.…as I outlined above.
I’ll try again….there are two triodes inside each of those 12A-7s. One of those 12A-7s is occupied functioning as the phase inverter. That leaves four triodes to function in two ‘channels’. Since you lost the Clean channel, that means that the two channels Do NOT share a common input stage triode. This is why the Gain channel continued to function. Pull V1….that Gain channel will NOT function, either, more than likely. Try it….we all are on a learning curve. Who knows…I could be wrong, but if I am then I am going to think that It is very hard to have an all-tube preamp that does one clean and one gain channel. if this experiment shows that the two triodes in V1 function separately as input gain stages for both channels, then do this. Reinsert V1 and assure that both channels are working. Then pull V2. What happens? Again…my guess is that the Clean channel may well continue to function while the gain channel will be dead. If this is so, we will have learned something about your amp. Or…we will have learned something other than what I am guessing is going on. IF both channels cease to function with V2 out, then we might surmise that both channel go through one of V2’s triodes with the sec9nd triode 8n V2 being applied solely to the Gain channel.


with this type of approach then IF sometime in the future the Gain channel ceases to function, you can replace V1 to see if the Gain channel triode has gone out…..as the clean channel triode failed you recently, right? Or….if that does not cure the problem ,you can replace V2 with a known good tube. The channel may come back up. If it does, then you know you had a bad tube there. If these tube substitutions do not return the amp to working status, then the amp has more serious problems in the circuit.
I hope this helps you understand your amp a bit better. I think it may.
fwiw, this is what a tech would do if you dropped it off. Hey, I do this in front of my customers so that IF they are caught in the middle of a situation where the amp needs to work,they might be able to install a tube or two and contNiue working/playing. Imho, D. Friedman could have quickly shared this information with you as an owner of his product. A simple signal path diagram in the manual would reveal these basics without compromising any secrets he thinks he has built into that circuit, and said diagram along with a simple narrative could have aided you in dealing with this problem in about two mI uses time.
Fwiw, there are very few secrets in any of these circuits when the circuit is being looked at by an experienced tech.
Thanks! That is some amazing info and analysis. I have always been interested in how a tube amp works, but like most things, until it's relevant it's not something I would just start reading about. Thanks to bad tube, it's relevant and I learned a lot. Appreciate the explanations.
 
Top