1969 Ampeg SVT Restoration Thread

[beans-on-toast]

That big brown paper covered cap is original and should be replaced. This is the preamp power supply cap.

I find that Dremel abrasive buffs ( 512E EZ Lock Finishing Abrasive Buffs 320 Grit (2 Pack) / Accessories Model: 512E) work well for cleaning the inside and outside of the tube shield chiminies. There are a few grits available. It is messy so I stretch a latex glove over them when working to protect the board. They come out looking like new.

Switchcraft jack washers and nuts are available from mouser. New ones look better than the rusted ones. The plating is thin on these and they usually don't clean up well. Better to replace those parts. If you replace the jacks, you need the long barel versions so that it properly extends through the panel.

Otherwise, it look very cleanupable.

 

[danfrank]

Hi. This is my first post, so, hello everybody. Did Wighat ever finish this SVT restoration?
I knew a bass player around 20 years ago that had 3 of these original SVTs. One was a 1969 with the 6550 conversion already done, the other 2 were a 1970 and a 1974. His 3 amps were in rough shape so we made a deal... I would go through each amp and change out all bad/marginal components and retube them to get them running in like new condition. As payment, he gave me one of the 3 SVTs and he kept the other 2.
Here is what I learned about the SVT while working on them:

1) The PCBs are the weakest link on the whole amp. They are made of phenolic material, which is basically thick paper coated with some sort of resin material. These boards SUCK! and contribute to the majority of problems that the SVTs experience. Unfortunately, this is what PCBs were made of back then. With the heating/cooling cycles of amp operation, these phenolic PCBs develop hairline cracks in them that sometimes will also crack the copper traces on the boards. I have looked for G10 fiberglass replacement PCBs, but never found anybody making them for the SVT. This would be a great money-maker for someone if so inclined: Make a set of fiberglass PCBs for the SVT, and instead of using 12DW7s, reroute the PCB traces so 12AX7s and 12AU7s could be used instead. I'm not talking about changing out component values so 12AX7 and 12AU7s can be used, but to reroute the circuit traces so these easier sourced tubes can be used. Good 12DW7s that aren't noisy are hard to find.

2)The SVT is like a 60s muscle car or high maintenance girlfriend... When things are working right, the SVT is an awesome experience to feel and hear, but there are just too many things that can get out of whack with it. It requires A LOT of TLC to keep it running right.

3) The 6146 is a TERRIBLE tube for this kind of abuse that the SVT gives the output tubes. The screen grids can't take the dissipation overload when the amp is driven hard or overdriven. The 6146 is a great audio tube, but only when amplifying clean, distortion free audio. Once the tubes go into clipping, the 6146s self destruct. The 6146 looks like a great candidate for an audio amp, it is really easy to drive, much easier than a 6550, and it provides a lot of output power. This is why 6146 SVTs can put out 350 watts easy at full bore while a 6550 SVT can barely do 300 watts. When the SVT started coming out with 6550s, the pre-driver should have been redesigned to provide more voltage swing to get full capability out of the 6550s. I really do like the low impedance cathode follower 12BH7 drivers for the output tubes, a nice design feature.

That's all I can think of right now. If more comes to me I'll write some more...
Thanks, Daniel

 

[JimmyM]

1. Can't say I've ever had a problem with my 69 directly related to PCB quality.

2. I don't find that to be true so much. Sure, tubes go out sometimes, but most of my problems with my 69 were related to issues it had before I got it due to the previous owner abusing it and for the 10 years it sat dormant at my house. And my 13 Heritage SVT-CL has yet to have a problem.

3. That's what Bill Hughes, the inventor, says, but I never run mine that hard so I'm keeping 6146b's in it.

 

[cerrem]

Daniel.... I don't agree with all of your SVT assessment.....
Once the SVT is gone through and made solid it is very reliable and in not need of much TLC ...
Yes the PCB board is made poorly... Not so much the Phenolic, but rather the copper traces are paper thin..... I did remake a SVT PCB with 4 oz copper traces and ground planes about 12 years ago using Mentor Graphics....only a small handful of people really give a S##T and I never made any fortune... I still have the Gerber files...
SVT with 6550 can put out the same or more power output...depends on the strength of the gm transconductance.... I have six 6550 tubes that are matched at gm of 12,800 and that amp puts out 360 W
The 6146 is much lower in transconductance and is less sensitive to drive...needs a bigger voltage swing to drive than a 6550... another reason the 6146 amps have more growl than its 6550 counterpart is because the pre-amp is turned up a bit higher to achieve the equivalent power output swing, thus more grind from the pre...
350 Watts from 6146 SVT is on a good day with wall outlet at 120V and all tubes at full gm of 7000 or greater....
6146 tubes can be robust and never have issues with them , provided the SVT weak spots are taken are of...
The trick to getting the most power from your SVT is to replace the heater wire with 12 or 14 Gauge wire on the output tube sockets ....and re-route the heater wire feed from the heater transformer to the center of the 6 tubes..... this will allow ALL 6 tubes to perform at their strongest....the stock heater wire is absolutely ridiculously tiny and makes for some major heater voltage drop .....by the time you get to the end of the heater string the droop is significant...
The other problem is the POWER ON switch in the front panel.... the 120V HOT lead passes through the fuse holder then to a tiny wire gauge to fit the MOLEX connector pin then up through the wire harness up to the ON switch then back down through the harness wire bundle and back through the MOLEX connector pin then to a post which THEN goes to the Power Transformer and Heater Transformer....this little wire carries the current for the entire amp!!! The voltage droop loss is there at idle, then at full power output the voltage droop is significant.. and you loose voltage on your tubes as well as more droop on the heater voltage....this all effects the power output significantly..
In the 69 heads there is a extra octal socket for the delay tube which is usually bypassed... Use this socket to install a relay for the 120V ...this way the 120V current for the power transformers does not pass up to the switch and back ... instead it goes to the relay after the fuse holder then to the transformers.... The 120V in the ON switch is only controlling the relay coil.....
As for the SCREEN resistors I usually use the flame proof resistors to keep the board from getting cooked....but flame proofs are not good at surviving over-load surge as carbon comps ...so 2 Watt works good... The diode clamp across the screen resistors helps keep the screen resistors from popping during intermittent screen current surges and forces the plate resistor to pop first before taking out a screen resistor... Just make sure to keep the Plate resistors up off the board by an 1" and make sure to use cement tub style..NO wire wounds, since these things are acting as fuses..
The reason most 6146 SVT amps constantly blow-up tubes and resistors is due to front end 12BH7 voltage amplifier is being fed from the 430V node....during loud transients and overloads this will produce an AC signal that far exceed the 12BH7 follower that has 220V on the plates... So when the follower grid is driven far over it's own plate voltage it saturates on the positive half of the signal and thus takes over the BIAS voltage forcing it very positive...then the time for this voltage to come back to normal is based on the time constant of the 150K mixer resistors and the coupling cap...by this time it is too late, the BIAS got pushed way positive and the current gets slammed through the 6146 and they blow...
Simply re-wire the 12BH7 feed same as on the later 6550 heads... Add a 1K and filter cap feeding from the screen supply to the front voltage amp of the 12BH7... AMPEG actually did this mod by cutting traces but very few 6146 heads had this factory mod since they moved over to 6550 tubes at this time and this mod remained in place on the following board revs with 6550 tubes.. Based on drawing dates, this factory mod was initiated in July of 1970...while the 6550 update was sometime in Oct 1970...
The biggest headache with these SVT amps is the pre-amp has nasty ground loops ...
The pre-amp has the ground wires all over the PCB board.... re-routing these leads can be a lot of time consumption... I cut ground traces and re-route in a STAR system to get all the hum out... The HUM is prevalent when the EQ switches are in a particular setting..

 

[beans-on-toast]

There are products available today called fusible resistors. They are available form suppliers such as Digikey, Newark, and Mouser. The screen resistors are intended to blow like a fuse when they are stressed. This is a good thing as it protects the amp. These fusible resistors work well here.

I usually don't install the diodes across the screen resistors. They are on the current SVT schematics with a note NP, they are not placed on the board.

Bill Hughes has acknowledged that the 6146B was not well suited to the SVT, the 6550 is a better tube to use.

Ampeg had a mod for the early 6146B based SVT amps. See timv's images Ampeg SVT by TimV attached here. Also see the TB thread: SVT registry.

 

[timv]

Yeah, I think mine is one of those between years with the factory mods. I remember finding the schematic differences for 6146's and there are I believe 3 different versions I have found.
I still haven't come across a 6550 equipped svt of any vintage that puts out more than 270 clean watts (no visible wave distortion on my oscilloscope) going through many ge's or whatever tubes and I have tried. Voltage held @ 120 with my variac.

My 70 with 6146's does 350+ easy.

There are products available today called fusible resistors. They are available form suppliers such as Digikey, Newark, and Mouser. The screen resistors are intended to blow like a fuse when they are stressed. This is a good thing as it protects the amp. These fusible resistors work well here.

I usually don't install the diodes across the screen resistors. They are on the current SVT schematics with a note NP, they are not placed on the board.

Bill Hughes has acknowledged that the 6146B was not well suited to the SVT, the 6550 is a better tube to use.

Ampeg had a mod for the early 6146B based SVT amps. See timv's images Ampeg SVT by TimV attached here. Also see the TB thread: SVT registry.

 

[beans-on-toast]

It seems that they were doing some scrambling, making some changes in the early days to deal with issues that came up in the field. It was an amp like none other at the time.

 

[cerrem]

One thing to note about conversion to 6550 on an SVT that originally had 6146.... The screen supply will be quite low at 220V and will make for a weak power output.... Ampeg changed the Power Transformer used in the 6550 version SVT had with a screen supply about 350V... So a trick to make for a real strong SVT is to rewire the screen supply on the older transformer to make a voltage doubler...you end up with about 400V on the screens instead of 220V .... Now you have an SVT putting out 380W to 400W ....Back in the 80's when tubes were not as expensive I did this voltage trick and loaded the SVT with real GEC KT88 tubes.... WOW!! that amp had punch !!!

 

[Chris 'Wighat' Jordan]

Hi. This is my first post, so, hello everybody. Did Wighat ever finish this SVT restoration?

I haven't really even started. It's disassembled, cleaned and labeled. I have gotten very busy with work and remodeling our house. Soon though.

Great information in all the recent posts, thank you everyone.

Oh, and @cerrem, I would be very interested in making a fresh PCB. Would you share your patterns and process please?

 

[Rickenbackerman]

I'd just like to go on record and agree with timv's comment about wattage - I own three early 70's 6550 equipped SVT's and have had 3 or 4 other ones come across my bench - never seen any of them make more than ~275W before clipping sets in.

I thought it was something to do with my test gear!

 

[GreaserMatt]

Holy grail of bass amps IMHO...

 

[beans-on-toast]

Holy grail of bass amps IMHO...

The B-15, V4B, and the SVT cover all the basses you can play.