Needing info on modding an old VM speaker amp.

[soundmasterg]

Yes, the 6AU6 should have plenty of gain for you, but it is probably setup right now for lower gain. To change it over to a higher gain circuit, the parts around it, mainly the resistors would need to be changed to different values. In order to do this though. I would need a copy of the schematic to suggest changes, and would also need to know the current voltages in the amp at various points. Any competent amp tech would need the same info. That schematic on the side of the speaker box is very helpful, but won't have the voltages on it....those you would have to measure with a DMM. The 6AU6 in this amp is the preamp. The 12AU7 is the phase inverter, though it could be a cathodyne inverter (basically the same thing, just a different type of circuit), in which case half of the 12AU7 would be the driver tube and the other half would be the phase splitter. The EL84's are the power tubes, and this is mono from all appearences, so it is probably a 10-15 watt amp. The big transformer there is the pwoer transformer, and is quite large for a small amp such as this. It could have been setup to run some other device through those octal plugs on the side of the cabinet. Your schematic may tell you...but you have to be able to read it and understand it. The output transformer may be inside the chassis, but you would have to pull the chassis to see. If you do so, be careful not to touch any of the circuitry inside until you safely discharge the caps. They may still have high voltages on them.

A competent tech is probably your best bet. You probably aren't in my area (Portland, Oregon) but if you were I'd be happy to take a look at it.

Greg

Thanks for all the info Greg. There is a web site dedicated to VM products where I could probably get the schematic. The one inside the cab will be "fun" to access. I'd try to have to take a photo of it and hope it comes out.

If this helps, the input for the amp is a mono RCA cord to the volume pot, which is connected to another RCA cord, which plugs into an RCA jack on the amp (grey cord on the far right in the 3rd photo). The smaller octal plug runs to the on/off switch. The larger octal plug connects to the pilot lamp, speaker, and the treble/bass pots. Tracing these wires was gross, as their covering is getting sticky and gooey (plasticizers migrating out of the vinyl).

Oregon is a bit of a drive. But I appreciate the offer!

Yes I saw that VM site also. If you search around a bit, you may be able to find that info without having to pay for it. The schematic inside the amp would be useful if you were able to get a good photo of it, and then post it in a readable form. A lot of old amps do get the sticky and gooey thing going on. Are you talking about the wires outside of the amp chassis itself or on the inside of the chassis? As long as the electrolytic caps have not leaked physically, then you are ok touching the insides of the amp. If they have leaked their electrolyte out, then that stuff is corrosive and the insides of the amp should be cleaned. You should also make sure to not touch the electrolyte yourself if at all possible. You should also make sure to not operate the amp until the electrolytic caps are replaced. If they have not leaked, it would still be a very good idea to replace all electrolytic caps in the amp. Age is not kind to them. Paper coupling caps also can degenerate, but these should be measured one by one to see if they are leaking DC on to the stages that follow them in the circuit. If they leak beyond about .3v then they shoul dbe replaced too. Any of this work can be dangerous inside the amp, so if you don't know what you are doing in order to be safe, take it to a tech.

Greg

[IHeartRics]

Yes I saw that VM site also. If you search around a bit, you may be able to find that info without having to pay for it. The schematic inside the amp would be useful if you were able to get a good photo of it, and then post it in a readable form. A lot of old amps do get the sticky and gooey thing going on. Are you talking about the wires outside of the amp chassis itself or on the inside of the chassis? As long as the electrolytic caps have not leaked physically, then you are ok touching the insides of the amp. If they have leaked their electrolyte out, then that stuff is corrosive and the insides of the amp should be cleaned. You should also make sure to not touch the electrolyte yourself if at all possible. You should also make sure to not operate the amp until the electrolytic caps are replaced. If they have not leaked, it would still be a very good idea to replace all electrolytic caps in the amp. Age is not kind to them. Paper coupling caps also can degenerate, but these should be measured one by one to see if they are leaking DC on to the stages that follow them in the circuit. If they leak beyond about .3v then they shoul dbe replaced too. Any of this work can be dangerous inside the amp, so if you don't know what you are doing in order to be safe, take it to a tech.

Greg

Thanks for the great info!

The sticky wires are outside the amp chassis up to the control pots. I have not removed the chassis from the cabinet, and glad I did not after reading this post. When I do, I will be careful! I assume the electrolyte from leaky caps look like a leaky battery? I've also heard that they can look swollen or deformed, right? I have operated the amp a couple of times in it's current state. It's not very loud, but pretty cool.

As for those octal plug, how do those work? do the wires run through them or is it some type of connector between the inside and outside on the amp? I was wondering If I could just jump the in-and-out that led to the treble and bass controls, as I just want a volume control like an old Fender Champ.

I defeinitely don't have that much depth of knowledge in amplification or electronics. I think what I'll be doing it trying to take it so far within my ability (taking the amp out and mounting into a small combo cabinet, maybe some basic soldering, etc) then take it to a tech.

[soundmasterg]

When the electrolyte leaks from an electrolytic capacitor, it is often a brown sticky paste. It is corrosive to the chassis and other parts, so you want to avoid that situation if you can. If the caps are original, you should bank on replacing them, just as insurance if nothing else. When an electrolytic caps starts to go bad, they often bulge on the positive end, but sometimes the look fine but perform badly electrically. Other times they look terrible but still check out ok electrically. New caps are MUCH better made than old ones, so any new electrolytic will be better quality and smaller than the old ones. It will also get the amp working the way it should be. The coupling caps as I mentioned before sometimes leak DC but sometimes they are fine too. Changing them can change the sound so I only change these if they are leaking DC signals to the following stage, or if they are otherwise compromised physically.

The octal plugs were usually used to connect between amp chassis and/or speaker boxes, etc. So there will be wires soldered into the octal plug and then that plug or those wires are meant to connect somewhere else. On the inside of the chassis, those connections on the octal plug usually connect to something inside the chassis. If all you want is a volume control, it could be done, but I would have to see the schematic to offer recommendations. Personally I would like tone controls on an amp if it was mine....they can be quite useful.

It sounds like you should take the amp chassis to a tech for sure. Things can kill you inside an amp such as this so you need to be careful. They can store voltages for a long time too, even if the amp is unplugged, so be careful poking around in there. If it was just the amp chassis itself that you wanted modded, those aren't too heavy....so with a schematic and an idea of what you wanted done, I could mod the amp for you. Even if you didn't have a schematic, I could trace it out myself if I had the amp here, but a schematic would make things MUCH easier. If you're interested in having me do something with your amp, let me know via PM and we can go from there. If not, then I am sure there are local techs around who could make something cool out of it.

Greg

[BAD RONBO, KiLLeR DWaRfS]

i have a VM amp that came out of a late '50's suitcase record player(has been in my family since new). 2x 6v6 a 5y3 rect and a lone 12ax7. played thru it for years. original tubes too. then i modded it with an xtra 12ax7 and tone stak. very gnarly.

[IHeartRics]

i have a VM amp that came out of a late '50's suitcase record player(has been in my family since new). 2x 6v6 a 5y3 rect and a lone 12ax7. played thru it for years. original tubes too. then i modded it with an xtra 12ax7 and tone stak. very gnarly.

Sounds really cool. Only inspiring me more but I need to focus on my current project and complete it first (a 1x12 closed back matching cab form my 1x12 Hiwatt combo - a mini stack for smaller venues ).

[BAD RONBO, KiLLeR DWaRfS]

i used to drive an EV 12" SRO speaker. sounded great. recently bought a VM model 8809-8810 amp from 1956 in a steel case. 1x 12ax7 and 1x 6v6 with a 6x4 rectifier. vol, bass treble with mike and phono input and tube balance control.

the suitcase turntable amp i was talking about a few posts ago was a model 556 from 1954. the pic on the VM site is exactly what it looked like .

[whojamfan]

Sounds really cool. Only inspiring me more but I need to focus on my current project and complete it first (a 1x12 closed back matching cab form my 1x12 Hiwatt combo - a mini stack for smaller venues ).

Love to see pics of that when you are done!

[IHeartRics]

Sounds really cool. Only inspiring me more but I need to focus on my current project and complete it first (a 1x12 closed back matching cab form my 1x12 Hiwatt combo - a mini stack for smaller venues ).

Love to see pics of that when you are done!

Will do. I can get to work on it now that I finished repairing an old Vox guitar case. Having trouble locating the black grill cloth used in the late 70's / early 80's. I've contacted several amp supply places but nobody has it or knows where to get it. I may just have to just get something that is close for now. If anyone knows please share the info!