Two Monoblocks to stereo, How To?

amplifiers, receivers and loudspeakers
Post Reply
gmm2511
United States of America
Posts: 4
Joined: 03 Nov 2017 03:11
Location: Dallas Texas

Two Monoblocks to stereo, How To?

Post by gmm2511 » 07 Nov 2019 03:48

Hi,
I have an old VM mono record player I am restoring that I would like to convert to a stereo unit. I have the original monoblock from the player and bought a twin of the same. I have no experience in pulling off this task but I am seeking advice for the safe way to do this from the experienced in this forum.

First, I believe the two units need to be powered by separate power cords. Is it feasible to connect two power lines to one switch to power them on together or might there be a junction block? Each monoblock also has a connection to the turntable switch to turn on the amp and turntable motor. How would I need to connect that?

I know I have to send the output signals from the turntable separated to each amp in which I already converted with a stereo ceramic phono cartridge and mounting the wires to RCA jacks I installed underside the TT.

There is no preamp so to speak on each monoblock but these did have a volume and tone control pots connected with capacitors and resistors via extension wires to reach the front of the cabinet, so I am assuming they are the preamps. I would need to change the pots to control each monoblock channel for stereo and am wondering what to look for. I have the capacitor/resistor rebuild kits which I am at task to do as well. I hope this is not something off the wall to do but I am sure there is a way. I am very good at following schematic diagrams, understanding most electrical internals and excellent at soldering. Thanks much for any help you can offer.
VM 560 MonoBlock.jpg
(19.96 KiB) Downloaded 71 times

aardvarkash10
long player
long player
Posts: 1410
Joined: 24 Mar 2009 01:57
Location: Auckland, New Zealand

Re: Two Monoblocks to stereo, How To?

Post by aardvarkash10 » 08 Nov 2019 20:54

Hi - really need a schematic and some close images of the top and underside of the amps you have.

What is your intended use for the system?

josephazannieri
long player
long player
United States of America
Posts: 2910
Joined: 20 Apr 2008 06:01
Location: Norwalk,Ohio,USA

Re: Two Monoblocks to stereo, How To?

Post by josephazannieri » 08 Nov 2019 22:28

Yo gmm2511:

I am somewhat in accord with A10, but, in my tradition of pontificating about things I don't have exact knowledge of, I will offer my best conscientious guess as to what you have. This thing looks like it has a relatively big power transformer, and a push pull pair of 6V6 output tubes with maybe a 5Y3 rectifier, and a 12AX7 preamp tube in each monoblock. I don't see the output transformers, though. I am assuming that you have a console with some speakers in it. My first good mono amplifier, back in 1963, was a similar 6V6 setup, and it sounded pretty good with efficient speakers. Now it will not be HIGH END, but it will likely sound good with a wide range speaker. You need to be sure that these are not "hot-chassis" amps with one side of the power cord connected to the chassis of the amp. Make sure that there are 2 transformers on each chassis. I think that you will find an output transformer somewhere, but it might be mounted on one of the speakers. The black transformer looks like a power transformer, and a suitable output transformer for 6V6s will usually be much smaller than that big black thing.

It appears that you have 2 complete mono amps with volume and tone controls on each amp, assuming that you have the output transformers. If you have put a stereo ceramic cart in your changer, you can hook each side through shielded cable to the input of one of the monoblocks, but you will need to figure out where the input is on the second amp. Just compare it to the original monoblock that you started with. See where the mono cartridge hooked up to the amp that you started with. As for getting power to the new amp, you should be able to to see where the changer hooked to the first amp and just use the same colored wire from the second amp and hook it to the same place on the changer.

With these best guesses, I am going to warn you to be sure that there is actually an output transformer on each of the chassis, or somewhere in that stack of gear that you have. You need to be sure that these amps are not "hot-chassis' units. Give us some more pictures of the chassis from different angles, and list the tubes on each monoblock. Then we won't be best-conscientious-guessing as much.

And good luck from the old best-guesser,

Joe Z.
i

aardvarkash10
long player
long player
Posts: 1410
Joined: 24 Mar 2009 01:57
Location: Auckland, New Zealand

Re: Two Monoblocks to stereo, How To?

Post by aardvarkash10 » 08 Nov 2019 23:20

^ All the points that had me bothered Joe.

Hot chassis is a significant safety problem that has to be identified and dealt with before anything else.

At 50+ years old, I'd be replacing all of the passive devices regardless of appearance or test. 6V6 in pp is a great start point for some basic tweaking and dragging into the 21 century. Judicious use of silicon can really lift the game of a 12ax7 / 6V6 combination to the point where, if the old output transformers are built to price rather than performance, a new set of better quality items would make these amps solid keepers.

Poinz's Musical Machine is pretty much the standard in this field, but there are plenty of other options.

I would keep them as monoblocks, put a nice timber frame around the old chassis and have them on display.

aardvarkash10
long player
long player
Posts: 1410
Joined: 24 Mar 2009 01:57
Location: Auckland, New Zealand

Re: Two Monoblocks to stereo, How To?

Post by aardvarkash10 » 09 Nov 2019 01:22

OK, found a sort-of useful schematic here

When I say sort-of useful, the emphasis is on the sort-of. However, it confirms that a power transformer is fitted and the chassis is not "hot". Also, the turntable is powered from mains, not as a part of the tube heater circuit as was common in the day. This is all good news.

Even better, the opt is multiple tapped so you can run a variety of speaker impedances.

The circuit is pretty straightforward - voltage amp up front in one section of the 12ax7, followed by volume and tone controls, on to a concertina phase splitter, then on again to the 6V6 outputs. The outputs are run in pentode mode and there doeen't appear to be any feedback in place so I'm picking it sounds a bit harsh through even a reasonably capable speaker. It will also have a relatively high output impedance and so will not cope with any standard issue modern speaker.

Some feedback (local and/or global) would certainly improve the sound and capability of the amps.

The power supply is similarly standard. Throw the tin can 3-way main capacitor in the bin and replace it with a modern set of similar spec separate items. Don't exceed the maximum capacitance spec for the rectifier tube - iirc its about 60uf max.

The power supply would respond well to tricking out with a set of low-noise silicon diodes replacing the tube rectifier, and better smoothing with larger caps and dropping resistors to better separate the three supply sections. I wouldn't regulate the power supply since it is feeding pentodes.

Yes, you can parallel wire the supply BUT you will need to ensure the ground for the power supply is well maintained and an earth lifter in each monobloc to separate safety ground from the signal earth will be required unless you have a taste for 60hz hummmmmmmmmmmmmmmmmmmmmmm.

FIT MODERN FUSEHOLDERS!!

aardvarkash10
long player
long player
Posts: 1410
Joined: 24 Mar 2009 01:57
Location: Auckland, New Zealand

Re: Two Monoblocks to stereo, How To?

Post by aardvarkash10 » 09 Nov 2019 02:33

... more thoughts...

Convert the finals to triode mode would give sonic advantages at the expense of raw output power.

Alternatively, ditch the tone controls that are taking up to 6db of headroom in the voltage amp/phase splitter section, and go with the same amount of "Schade-style" local feedback around the finals in pentode mode. Look up the Baby Huey amplifier design for inspiration.

Either of these options would make use of the current opts and would overcome the typical shortcomings of mass-produced transformers of the era, while lowering the output impedance and so making matching to more modern speakers easier.

gmm2511
United States of America
Posts: 4
Joined: 03 Nov 2017 03:11
Location: Dallas Texas

Re: Two Monoblocks to stereo, How To?

Post by gmm2511 » 09 Nov 2019 04:15

Hey guys, thanks much for your ideas. I’m actually in London currently, 4AM here and have to catch a morning flight home so I’m off to sleep. I will send photos and schematics when I get back. For reference, this is the well known VM 560A mono record player. Got it from my father-in-law when we moved him. I want to modernize it and make this sucker rock better than original. I love these old players from the 50’s and haven’t counted lately how many I have. Anyway, thanks again and I’ll be back here either later today or tomorrow.

josephazannieri
long player
long player
United States of America
Posts: 2910
Joined: 20 Apr 2008 06:01
Location: Norwalk,Ohio,USA

Re: Two Monoblocks to stereo, How To?

Post by josephazannieri » 09 Nov 2019 05:05

Yo gmm2511:

I agree with our Australian campadre A10 that this record player is NOT a hot-chassis unit, so you will not have to live in fear of frying yourself with it. In your original post, you said that your goal was to "restore" it, and you have already purchased a ceramic stereo cartridge, so I think you want to make it into a stereo player by hooking up the second amp and some speakers to the second amp. I get the impression that you are not interested in modifying it extensively, but rather having a cheap'n'cheerful (but safe and good-sounding) old-fashioned record player. I am in the U.S like you, and I have discovered that there is a SAMS Photofact manual available for this thing, and I suggest that it might be worth purchasing. Here is the link to SAMS publishing:

https://www.samswebsite.com/en/photofac ... /brand/V_M.

I LOVE SAMS Photofact service information. You will have to look at your unit and decide if it is a 560A, B, C, or whatever. The prices are reasonable and you will get a really clear, correct schematic and a lot of service info for $15.00. The schematic unit supplied by A10 has 3 speakers, two 2 5x7s and one 6x9. From the schematic supplied by A10, it looks as if each speaker is on a separate winding of the output transformer. I have an old Williamson mono amp that is similar to the 560 that you have, and it puts out about 12 watts, and it will make any efficient speaker jump. You will get speaker impedances and everything else you need to restore this thing, and it will cost you $15.00. You may have to replace the filter capacitors and other caps.

I have also discovered a numbers of YouTube videos of these things playing. You should check and see what you have for speakers. Some of these came with stereo cartridges, and some of them had 8 inch speakers with tweeters. What do you have in yours? If I were in your shoes, I would build a wooden box the same size as the original cabinet and put my second amp and speakers comparable to the ones in your present unit in the new box. Also there is a rebuild kit available which includes all the capacitors and resistors necessary to rebuild your unit available on EBay for $59.95, found here:
https://www.ebay.com/itm/Voice-Of-Music ... SwdzVXjTeR

You get a schematic with the rebuild kit, but you don't get the service info. And good luck from that cheap'ncheerful old guy,

Joe Z.

Post Reply