Schecmatic for Ampeg SVT RV

[km6xz]

Hi Forum
I just received a SVT RV that is a little different than all the other output and bias circuits I've seen in SVT's. The circuit is pretty straight forward but I am the second tech to look at it. The first one changed all the resistors on the power amp sub board. and when they smoked due to being quite off in values and having KT88's instead of 6550's. I am assuming the screen resistors are supposed to be 220 instead of the older versions that used 22 ohm. The older versions also had plate inductors but this one has 6 open and crispy film resistors. The grid resistors were not burned but were obviously not close to the typical 47k at 470k. .....a long story short.....I have no print of this to see what it is supposed to be.
Thanks a lot, in advance...

[J M Fahey]

Dear km6, don't know that specific model, but in a general view, "a tube is a tube".
Smoked resistors mean too much current through them.
Are they in series with some tube element?
Then, why is that tube passing so much current?
In principle I'd pull them, check all the voltages are what they should be, specially bias because it controls *all* other currents (meaning cathode, screen, plate and even grid), plus the obvious: does bias voltage actually reach the grid?
Of course there is the high possibility of having a bad tube, which kills all others by killing bias voltage.
After checking, I would bias it *real* cold (as in practically cutoff), plug just one new known good tube, see what happens, and add them one by one.
If no sparks or nuclear clouds, I'd slowly and carefully bias it hotter until everything fine.
Good luck.

EDIT: *why* 470K? Use a sensible value, whatever the schematic asks for.
Maybe the former technician was as long sighted as me.
(Not a typo, I can see faraway things very well, problem is that anything I can actually reach with my hands is blurry)
Shortsightedness *may* be a blessing in this profession.
Oh well.

[km6xz]

Hi JM
Thanks foe the reply.
I am was just trying to start from original values and not run it with tubes in place, since these KT88s are too valuable to experiment with to determine optimum values. I put them on my curve tracer and found these must be new, installed just before the other tech gave up on it, so I just put them aside. The bias detection circuit is different on this model also but looks as if it was left unmolested. On this version, one cathode current detection sample for a bank of tubes is sent to the comparator, instead of each cathode as in other post 1970 models. The "poor" kid who owns it paid $3000 for it and it came back to him missing all the knobs and 3 of the controls so stiff that they do not turn without pliers, and instead of a complaint of low but undistorted volume corrected, it came "modified" to fix it until the tech gave up and said it was not repairable.
I hope someone has the diagram for this model so I can put it back in original state before troubleshooting the original problem which has to be minor. They are heavy but SVTs of any stripe are pretty simple and logical devices. I hate working on gear that has been modified unless it is done with reason and logic and good engineering principles. The original SVT with 6146A worked reliably only after mods but the 6550 era established the model as THE bass rig if someone could handle the weight....mass and cost. When I had a large shop in California we had a few come in everyday from around the world after the word got around that we knew them and how to care for the model. One day George Clinton sent over 2 dozen to us to be brought up to spec during a one day break towards the end of his tour.

[J M Fahey]

Wow!!
Still hope somebody has the schematic of that particular version (hint hint, Enzo, pdf64, Frondelli, the usual suspects he he) but meanwhile, why don't you spare those KT88s and roll it back to standard sturdy and available (specially in Russia, ahem !!) 6550s ?
I much suspect your rich kid client (I do have those too, and deal with them around once a month) asked for KT88's "because they are the best" and for no other real technical or sound issue.
Probably now he's so scared to lose his amp , plus he must be missing it, that he will become reasonable enough to let you do your work as you, with your experience, see it fit.
Don't think he will balk at paying for a new set of 6550's , under the circumstances.
In fact I would buy, say, 8 of them (or even 10) so as to hand match them and give him the selected best.
I often do that for my rich clients, order 6 power tubes for tube amps that need 4.
Often I find 4 very good ones, 1 reasonably good, 1 weakling .
We are talking JJ and such.
Good luck and enjoy St Petersburg.

[Steve L]

Do you mean an SVT VR, instead of RV?
If so, here are the schematics:

Ampeg SVT-VR (07S365) Power Amp Schematic.pdf
Ampeg SVT-VR (07S792) Power Supply, PI Schematics.pdf
Ampeg SVT-VR (07S598) Preamp Schematics.pdf

Steve

[km6xz]

Thank you very much Steve! Yes, my mistake reversing the suffix letters. This is exactly the one I am looking at. This is the first of this model I've seen since leaving the business 7-8 years ago and my intent was to put it back to stock before attempting to solve the original complaint. The differences are minor between this on and, say, the SVT II Pro, but enough to want to have the values closer than my calculations. As it turns out my calculations were pretty darn close.

JM: I've discovered a little more about this amp and the "kid". He came to me with his friends originally last week when the friend brought a CR1604 Mackie with 4 channels down, and lots of intermittent faders and one broken one. The problems had accumulated for years because they did not know of anyone who could fix it. The one shop in the city has a terrible reputation and has a flat rate that for a mixer over 12 channels of 250euros labor. I was able to repair it the next day and they could not believe that anyone could or would. I charged his friend 400 rubles, about $13, for parts. I did not have the sip-9 chips but had the 8 pin and added a jumper for positive potential. The kid with the Ampeg begged me to look at his amp, he just got it back from a "master" as craft technicians are called here and it could not be repaired.

Last night, when he brought over the "original bad tubes", 6550's upon my request I learned more of the details. He is not a rich kid, but a young looking 24 year old bass player who works as a dentist as a day job. Since he works in a clinic as part of the national health care system his salary is not high so he saved 3 years to buy a real SVT. The only one he could find was a new one in Moscow 2 years ago. He was concerned that this new one was build in Viet Nam and not a "real one". It was $3000. He also bought the big retro Ampeg cabinet. Having no rent or mortgage(the majority of college students, which means almost any one under 20 here in St Petersburg, owns their apartment free and clear) so can save a lot of his income for discretionary expenses and instead of a motorcycle or $1000 iPhone or travel, he saves for bass rigs. Last night he said the "master" had his amp for 1 year before giving back to him non-functioning. The "master" managed bang up several pots so they are frozen and it had no knobs. The power amp sub board had ALL the parts changed with large solder blobs everywhere. It had 6 new KT88. I asked to see the originals that had been working before the problem and he brought those last night. They were Sovtech 6550s and had barely any ink discoloration and no internal contamination. I tried 5 of them(the 6th was not there) with my curve tracer and seemed to fine.
If there was a part source, a shop which was honest and properly equipped, could make a very good business here. I would consider it if I could find a large shop in the US who would send me a box of original parts on a regular basis. The manufactures will not send parts when the national distributors are supposed to, but don't.

[J M Fahey]

Yes, my mistake reversing the suffix letters.

Well, you can be forgiven for *that*.
If *all* Russians invert their "R" (ja), why wouldn't you invert the whole "VR" suffix?
At least you didn't replace "V" with a "B" he he.
Back to your customer, glad he found you.
As of the monthly "box of parts", can't you arrange something with your ex-technicians?
Good luck.

[km6xz]

;>) Maybe that is the problem, I speak Russian poorly, being a mono-language Californian.....
My old shop was sold off by the employees I gave it to a few years ago and the new owners quickly went out of business. I wish I still have the schematic library, tons of test gear(...mostly HP, Tektronix and Sound Technology) and parts warehouse. Unfortunately the US based brands are not at all concerned with customers in other countries and allow their national distributors rip them off in the name of the manufacturer. IF some one from upper management of any of these companies were to spend a few weeks visiting several countries which have great sales potential they would change their lazy and non-responsive attitude. They are leaving the door open for new brands out of China to take all their business. Sales would double without spending any money if they just created stronger oversight over national distributors. The market here for example is gigantic, 100,000 musicians in this one city alone who have formal educations in music, and with money to spend but lack of stock, no repairs, no parts, no product loyalty or knowledge by the stores etc....Only one company has a policy of allowing end users to get parts if the national distributor refuses to release parts: Peavey. But the distributor tries to get people not to buy Peavey because they loose control if the user can keep their gear working for a long time into the future. One store 2 blocks away has Ampeg, Marshall, Fender etc but at double the normal price and come with no tubes. A pair of 6L6GC's will cost $100 extra to install in the amp that normally comes with tubes.

[MarkusBass]

If there was a part source, a shop which was honest and properly equipped, could make a very good business here. I would consider it if I could find a large shop in the US who would send me a box of original parts on a regular basis. The manufactures will not send parts when the national distributors are supposed to, but don't.

Hi km6xz, what kind of parts you need? In most cases I had I was able to buy parts either in Poland, Germany, or UK. The worst case is with parts that cannot be easily replaced (e.g. Ampeg potentiometers). But in this case you can e.g. contact Ampeg service in Germany. If you list what you need, I would try to tell you whether you can find these parts in your neighbourhood. I assume that there are no problems with valves in Russia.

Mark

[km6xz]

Hi Mark
I have not been able to get a source for common OEM parts for the major brands, like keybed parts from Roland and Korg, parts from any modeling amp company such as Line 6 (Although while in California they sent parts to me for free since I was one of or their first warranty station when they first started), Berhinger, parts for DJ cd decks and TTs. The ONLY manufacturer to send parts without a problem has been Peavey.
Generic parts like semiconductors, and passives are not a problem, in fact I've gotten some pretty good prices out of Moscow but the minimum order is 5,000 rubles or about 130 euros.
By the time any parts are ordered, shipping it 10 times the value of the parts from the US.
Tubes are a real problem unless NOS 6п3с-е (6L6 type tubes), because none of the Russian companies have distribution in Russia. I have been buying directly from the largest tube plant in China and having very good luck. I've approached two western brands about becoming the Russia distributor but there is no interest in having one. We can get tubes directly from Svetlana(the real one) here in St Petersburg but they are just about out of the business off small tubes, and keep raising their prices between every rare batch. They are not a reliable supplier anymore, very seldom having any EL34's or 6550s. They have not had a rin of 12AX7s for 6 months. If someone wants a low cost but reliable 6L6 style tube local tubes are readily available however. Also a 6.3 volt 12AX7 style tube is also available from old Soviet stock. Metal anode transmitter tubes are cheap and good but not everyone wants a 600 watt single ended amp or has the power supply to power a 5,000 plate dissipation tube despite costing only $40.
I had the plan of finding parts suppliers in the west and stock larger quantities, and set up a network of affiliated shops in major cities and supply parts(the only thing keeping techs from opening repair shops here) with a central ordering and work order cloud based web application so the network could be kept tight and controlled. Then it would make sense to order from a number of sources each order would be large enough to have shipping a reasonable portion of the total cost to get them here.
Are there any sources in Europe who will set up wholesale accounts outside their country....and ignore the restrictions on importing parts other than through the national distributor?

Edit: The original reason for posting was the schematic on a butchered SVT VR. I put it back stock and all turned out very well. The original problem was surely plate resistors being 1 mm from the traces under the resistors...which were cathode returns. Arcs, sparks and smoke was the natural result. That was an assembly problem, the 3 watters needed to be 10 times that distance. But the tech who tried to modify it to make it work obviously never found that problem or had any business working on such an amp. So all my work was simply putting the unit back to stock and mounting the resistors in the proper way.
Stan

[MarkusBass]

Stan,

I work with less sophisticated equipment (mainly bass and guitar amps like Ampeg, Eden, Marshall, Trace Elliot and similar). I know that there are service centers for Korg or Line6 in Poland but I have no idea where they get parts from. Have you thought about establishing a service center network in Russia and bypassing the national distributor (if this is possible)?

Mark

[km6xz]

Hi Mark
No doubt the national distributor in Poland is more ethical and allows parts to get into the hands of shops and end users. There is no pro audio repair industry here now because of the monopoly of parts and the 2 distributors who import 95% of all gear refuse to sell parts or repair anything out of warranty, telling people their equipment needs to be replaced if out of warranty. There are a couple shops in Moscow and one in St Petersburg but the one here has a well-deserved bad reputation and high prices. If something breaks while under warranty it will have to be shipped to Moscow and there is a 30 day minimum wait before it can be looked at. So naturally that does not encourage people to have things repaired.
With so many musicians and club/theaters using so much pro audio gear it still surprises me how no support industry has developed. The musican forums have listings of repair, most are piano tuners and luthiers with only two posting, working under the table from their apartments, doing electronic repair. One of those is me although I am primarily doing it as a hobby right now. I just moved to a larger modern apartment and have a 25 sq meter room set up for my 3 meter long main work bench with 26 pieces of nice HP, Tek, Sound Technology, and Fluke Lab series test instruments. I also have a Motorola Communications Service Monitor for doing all sorts of RF testing and analysis. It has 21 instruments included such as 1000mhz spectrum analyzer, tracking generator, loads, modulation meter, tone generators, 140 db signal generator output range... so quite likely I have the only gear in the city capable of doing wireless guitar packs and mics.
So it appears that there is an opportunity to create a support infrastructure that is needed and fits my interests and experience.
Stan

[MarkusBass]

This is strange. Do you mean that they throw away amps that they cannot fix? They must be very cheap in this case. Have you thought about exporting such stuff to neighboring countries (and geting them fixed there)?

Mark

[J M Fahey]

Dear km6.
I think you may break the rules by following them, if you know what I mean.
I would set up a "virtual" km6 service shop again in USA, you'd need a friend or partner (who would get some $$ for his efforts) with a physical address in USA to be able to get the parts packages, pay for them from a US account, everything for the business to appear fully within US , as not to irk international representatives.
In a way, "back to the old times".
Thanks to Internet, you can exchange EMails as if they were from inside US too, so nobody's the wiser.
All the manufacturers need to know is that "km6 from California" is back in Town, ordered some parts , which were shipped to his shop there, he payed from there, and can be reached at a certain EMail address reliably whenever necessary.
You wouldn't be making regular fixed-price warranty service , which seems to be a dying art form anyway, but customer-payed after-warranty-ends service; you'd just need parts supplied, which you would pay through the nose, (at service shop prices, of course) ,don't think they might have too much trouble with that.
The fact that Mr km6 is "outside on tour with a band doing their maintenance work" or something equivalent is irrelevant for them, they needn't know anyway.
I agree that Post costs are expensive on parts, but you can wait until you fill, say, 6 or 20 pounds worth of them or whatever weight/size is $$$ efficient.
Yes, it might delay deliveries up to, say, one Month.
Who cares ?
In the particular situation you are you still will be the fastest horse in town.
Or the *only* horse for that matter.
Maybe it doesn't justify shipping transformers or speakers, but for other parts it should do.
Have you checked USPS international prices for packages up to 20 Kg?
The heavier ones start to get reasonable, approaching classic U$S 7/Kg.
Or you might make some package deal with DHL or similar Courier.
Don't know whether you'd have problems with Russian Customs, though.
just my 2 копейка

[km6xz]

I am not sure where they are, but people will not buy a broken anything. Used prices are high, the idea makes more sense to import used clean equipment because lighter pieces are a lot cheaper in the US plus their shipping leaves a profit after selling here for higher used prices. Same with cars, used cars are very expensive and new cars are about 40% higher than in the US. Each person for whom I have repaired something has commented how they were surprised it could be repaired, they are just not used to it.
I have talked to dozens of stores to see if any would take in units for repair and I would come pick them up, and return them allowing them to mark up the repair. None was interested, they do not want units repaired and say their customers would not trust a repaired unit to ever work right again, no, the only answer for them is buy a new one. That would seem very strange in a country where there was a support industry, and a large backlog of functioning older units from decades of accumulation but that is not Russia, most buyers never had any amp before, there really are not a lot of older amps, pa or studio items in existence here. Retail stores with American and Japanese, and now, Chinese, equipment is a pretty new situation. Stores are everywhere now however selling all the well known brands, at high prices. None of them stock tubes, although a number of people sell them from their apartments by mail order. If one needed a quad of 6550s or EL34 today, in this city of at least 100,000 musicians there is no way to get them. That is why I created my own brand, mostly Chinese, who are easy to deal with compared to Russian producers. I test them and print a graph of curves each one, not just static plate current like the common brands. Matching is more accurate and a better predictor of how they will perform in a circuit with other tubes. My first batches are graphed by simply taking a photo of the screen of my curve tracer but I am almost finished with an analyzer which prints directly, with a section for serialized label, warranty, specs and serialized box sleeve.
I did not want to get into the tube business but I needed tubes and none of the companies were interested in importing to Russia unless at retail prices.
Stan