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Thread: RCA 6L6 Mono Amp # MI-6599

  1. #1
    Member HaroldBrooks's Avatar
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    RCA 6L6 Mono Amp # MI-6599

    Need help with schematic for an RCA 6L6 mono tube amp, model (or chassis not sure) MI-6599

    Could have been part of a PA or perhaps even an ancient TV, not sure.

    Here a pic of one I found doing a search, came up empty on a Schematic https://www.worthpoint.com/worthoped...-amp-154576780

    This one is a tad complicated for me, as it's not setup like a guitar amp per se, and there is a huge cluster of wires. I checked the transformer and all seems well, as do the heater voltages to all tubes, and the B+ is right around 400vdc with just the rectifier connected. Off to a good start in any event.

    Thanks for any and all assistance !

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    Last edited by HaroldBrooks; 12-06-2019 at 03:06 PM.

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    Senior Member vintagekiki's Avatar
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    Who does not know and knows that he does not know - teach him Confucius)
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    Supporting Member Jazz P Bass's Avatar
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    Tube lineup:
    1) 6L6 x 2

    1) 6SN7GT

    1) 6SK7

    1) 6J7

    1) 5U4G

    A chassis shot and a gut shot:

    Click image for larger version. 

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    Click image for larger version. 

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ID:	56284

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    Last edited by Jazz P Bass; 12-09-2019 at 04:26 AM.

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    Member HaroldBrooks's Avatar
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    Quote Originally Posted by vintagekiki View Post
    Thanks for what you added, it helped a bit. I decided to take this one as a "project" and I believe I will need to strip out some of the exotic Hi-Fi circuits in favor of a more simple guitar amp circuit. I've already started on it, and so far I have the power section working quite nicely. I get sound from the PI tube grids, so I believe I'm OK up until that point, but something in the preamp section is out of kilter, and I will need to strip away a lot to rescue that section. Lots and lots of capacitors in this amp, and I am not used to the exotic filters, and Have a feeling they will not help at guitar sound much.

    It's going to take some time for me without a schematic, but I am un-deterred !!! Onward to make this gem into an equally good guitar amp !!!

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    Member HaroldBrooks's Avatar
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    Quote Originally Posted by Jazz P Bass View Post
    Tube lineup:
    1) 6L6 x 2

    1) 6SN7GT

    1) 6SK7

    1) 6J7

    1) 5U4G

    A chassis shot and a gut shot:

    Click image for larger version. 

Name:	RCA chassis.jpg 
Views:	24 
Size:	260.9 KB 
ID:	56283

    Click image for larger version. 

Name:	RCA MI-6599.jpg 
Views:	33 
Size:	366.7 KB 
ID:	56284
    Thanks for your help, but there is a complication for the preamp section and I think they were using two channels with different sensitivity and filtering. That is the 6SK7 and the 6J7 I believe are both used as preamp tubes for different purposes, one for a hi-fi input, the other for a microphone. I will try to maintain using both of those tubes for two separate inputs, or perhaps run them in series to get a more layered guitar amp distortion. We shall see !

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    Lifetime Member Enzo's Avatar
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    Exotic? This is a pretty basic amp really. Starting with drawings of the tube bases, you could trace out the schematic fairly easily. I have done that many times for old amps lacking documentation. This predates most fancy circuitry. I could almost draw it from the photo.

    All those wax covered caps will need to be replaced. Your modern caps will be much smaller physically. SO plan ahead. Some larger caps including lead wires were 8-9 inches long. A modern film cap maybe 2 inch. SO plan ahead with wires.

    There is a grid cap missing on the one 6J7, that would certainly stop sound from coming through.

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    Member HaroldBrooks's Avatar
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    Thanks Enzo, I installed the cap atop the 6J7. Exotic is a relative term, as I am currently used to working on comically simple valco amps, almost exclusively !

    Question : there are two big blue colored what appear to be resistors leading to each 6L6 screen. One was attached to the rectifier pin #2, the other is attached to ground, and at this point, I am not 100% sure they are resistors since I couldn't get reading on either out of circuit. I went a simpler route and connected the screens to a second filter node separated by a 5.6k 5w resistor, and into a 33uf filter cap.

    I will probably have more questions soon... Thanks for your help.

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    Last edited by HaroldBrooks; 12-10-2019 at 11:48 PM.

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    as it's not setup like a guitar amp per se, and there is a huge cluster of wires
    So why not remove all the wiring and start fresh?

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    Member HaroldBrooks's Avatar
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    Quote Originally Posted by dmartn149 View Post
    So why not remove all the wiring and start fresh?
    Thanks for your suggestion, and that went through my mind, but I also revere certain things that might have been done already in the design of the amp, perhaps to minimize noise and hum, so I am a bit reluctant to scrap everything... I am of a mixed mind with this !

    With the last three Hi-Fi type amps I purchased and converted to guitar amps, I got away with leaving most things intact, and just changing a few values of caps, resistors, and replacing the obviously dodgy stuff. I would first like to jump ahead just to see if everything works in the chain, so I might do an alligator clip of the preamp tube plate with a coupling cap in-between to the proper grid of the PI tube. Now I just have to figure out what grid that is on the 6SN7GT. Both of the preamp tubes have plate voltage and heater current, So one would assume they should work if the rest is in order. I will try and let everyone know.

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    Senior Member vintagekiki's Avatar
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    Why remove wiring and start fresh.
    You should make an effort to keep amplifier in as much original (vintage) condition as possible.

    If you already decide to remove wiring, then remove and replace all components except transformers and make new (desired) concept of guitar amplifier.
    Before or later it will be time to replace octal preamp tubes, so when you work, replace in time octal preamp tubes with noval tubes.

    And what remains in the end. Metal chassis, transformers ... Sad.

    Advice. Make every effort to MI-6599 stay in his original state. Vintage is vintage.
    Old men deserved it.

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    Lifetime Member Enzo's Avatar
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    Philosophically, I think it is a choice between converting an amp to guitar use, or building an amp in the chassis. I like conversions because it requires me to understand the circuits at least to some extent, and alter it in ways that work for me. I might learn something. If you gut the amp and build a 5E3 in it, you might wind up with a nice amp, but it is just another 5E3, and offers no new challenges.

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    Don't forget the joker g1's Avatar
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    Those blue bits to the screens look like wirewound power resistors to me. Double check resistance with your meter and closely inspect for any markings.
    As described, they form a voltage divider to drop the B+ and supply the screens. (2 resistors in series from B+ to ground, with screens connected to the junction)
    But because of the size of them, I think they are also running a bit of current through them which loads down the supply somewhat. So with them removed, you may end up with more B+ than normal. Be wary of the voltage on the supply filter caps.

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    Member HaroldBrooks's Avatar
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    Quote Originally Posted by g1 View Post
    Those blue bits to the screens look like wirewound power resistors to me. Double check resistance with your meter and closely inspect for any markings.
    As described, they form a voltage divider to drop the B+ and supply the screens. (2 resistors in series from B+ to ground, with screens connected to the junction)
    But because of the size of them, I think they are also running a bit of current through them which loads down the supply somewhat. So with them removed, you may end up with more B+ than normal. Be wary of the voltage on the supply filter caps.
    I think I ran into just that type of problem when I got the amp up and running tonight. I added some more resistance to the screen power supply node, and that seemed to quite things down a bit. I couldn't measure any resistance despite my best efforts. The transformer on this one is Thick, and it doesn't seem to heat up at all, so I might just add a couple more resistors at the same points, or perhaps screen grid resistors. The DC voltage at the screens after I added a big resistance at the screen supply was 332vdc. Not too much I believe, but I don't have any experience with 6L6s, just the litter tubes so far.

    Thanks for the great input !

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    Lifetime Member Enzo's Avatar
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    You should get rid of teh old metal 6L6, it really isn't a great tube for audio, and it has lower voltage ratings than the glass ones. meanwhile common current production 6L6GC or similar regularly run at 500v in amps.

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    Member HaroldBrooks's Avatar
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    Quote Originally Posted by Enzo View Post
    You should get rid of teh old metal 6L6, it really isn't a great tube for audio, and it has lower voltage ratings than the glass ones. meanwhile common current production 6L6GC or similar regularly run at 500v in amps.
    I think that is the problem here, and you are correct.

    I have quite a few 6V6 metal can tubes, and they work splendidly in a couple of my Valco amps, but I believe this amp is stretching the limitations of the Metal 6L6 with the significantly higher plate and screen voltages.

    Just ordered another Glass JJ 6L6, and when that comes the metal 6L6s will be put out to pasture. I don't like dealing with marginal parts anymore, I can build in a compensation for the tubes, but after a while, it raises the question why ? and I think the answer lies in my nostalgia about certain old tubes, along with those same tubes sounding better in "some" of my amps, but then the reality sets in that some of those older tubes have limitations that newer tubes have shed, particularly running higher plate and screen voltages.

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