I have a hard one for you guys

[Hons Gruber]

Hello,

So today I picked up a new bass amp "well new to me"

It is an SG bass amp. They were made from 1973-1974 at the old Standel plant in El Monte, California in 1974 when Chicago Musical Instruments (CMI) purchased Standel. Anyways it was another amp associated with Gibson.

The amp is a 100 watt amp. It has 3 tubes 2x8417 and 1x12AX7A. It has one 15" woofer and two 10" woofers. It weighs in at 148 lbs.

I got it for the song of $50. The complication being there were no tubes in it. There are two reasons that I see that these do not sell for a lot. One is the 8417 tube is rare and hence expensive and almost nobody wants to replace that including me. The other being it is so very HEAVY...

Anyways so here is my dilemma I am assuming all of the electronics are good, but I will take the amplifier unit of hopefully tomorrow to get a look inside. I don't know that the electronics are good. I also don't really at all want to shell out a $100 on these rare tubes for one to blow quick. I have heard they last a long time, but still they will be very very hard to get in a few years.

So here is my goal which where you guys can hopefully help and shed some light. I want to convert the tubes used to something more common and more replaceable. How do I go about this? I have heard it is possible.

I realize to do this I will have to probably change around some electronics. Could it be done though? If possible I would also be able to do it without losing too much output?

Thanks guys :-)
shalom,
Matthias

[Svein_B]

With a liitle help from Google:

Circuit diagram for a QS with 4817: 8417 Amps

Conversion to 6550: http://www.triodeel.com/8417.htm


SB

[Gilgy]

Originally Posted by Svein_B

With a liitle help from Google:
Conversion to 6550: http://www.triodeel.com/8417.htm

Odd that he recommends discharging the 320uF caps "with a screwdriver"

[SemperFi]

LOL. A 320uF cap charged to over 400v! How many Joules is that? Isnt it C times V squared!? I could use that trick on a small cap, say 1u, but a 320uF? At least he warns 'there might be a spark'. I assume there is a bleeder resistor in this, or he probably wouldnt say that.

[Alain Poitras]

Yes they are rare and expensive ...

The Sylvania 8417 datasheet.

This datasheet say for 100W class AB1 typical operation, the circuit need a 560V supply for the plate, 300V for the screen grid, 4200 ohms PP for the load, etc ...

Maybe, you can look at others tubes datasheets to find the closest that can work with those voltages and this load. You will probably have to modify the pinout and bias if you can find one but most other powerfull tubes like that are very rare and expensive too.

If you got enough room to add two big tubes, you can use two 807 in parallel to replace each 8417 and obtain around 90W. You can get them for about nothing on eBay ... But you will have to change the sockets and they have a pin on top for the plate. Here is the datasheet :

The RCA 807 datasheet.

In class AB1, with 500V for plate, 300V for screen grid and a 8200 ohms PP load you can get 46W, then for two 807 in parallel, around 90W with a 4200 ohms PP load ...

In class AB2, with 500V for plate, 300V for screen grid and a 4600 ohms PP load you can get 75W, but the driver must be able to supply the 0,2W to drive the grid and you will have to add adequate driver tubes because the 12AX7 is not made for that.

In any case, the circuit have to be modify and some tubes added. You are lucky to get this amp for 50$, just the power and output transformers and the speakers worth over 400$ for sure, you have to do something with this amplifier ...

I just found is schematic, it is a hybrid with a solid state preamp and a tubes power amp ...



Source : http://www.prowessamplifiers.com/schematics/images/sg100.jpg

The first step is to check if the solid state part work well and the amp don't need any tubes on to do that. There is a jack name "SIG LINE" on the amp (At right on the schematic), just plug it on a auxilliary input of any sound system and try it ... Then you will know if this part is OK before going further.

Happy Halloween !

Alain.

[Alain Poitras]

Originally Posted by Svein_B

With a liitle help from Google:

Circuit diagram for a QS with 4817: 8417 Amps

Conversion to 6550: http://www.triodeel.com/8417.htm

SB

You are right, the 6550/KT88 is a better choice than the 807 because only two tubes are needed to get 100W with 600V plate, 300V screen grid and 5000 ohms PP load in class AB1 ...

The Tung-Sol 6550 datasheet.

And the 6550 cost much less than the 8417 and is not rare ... A very interesting tube.

Alain.

[toprepairman]

The spec sheet reads 35W max anode dissipation, but the circuit shows 465V at 100mA which is 46.5W so it's being rather overrun, anodes might glow....

[Original Burnedfingers]

You should be able to throw a set of KT88's in it and fire it up. Check and set the bias. You will probably need a slight modification to get the bias correct. In my opinion based on the Quicksilvers I have owned you probably will not need to parallel up the input stage per the conversion information. I did a Quicksilver with nothing except a grid resistor, coupling cap change and a slight modification to the bias circuit and it had NO trouble in making its intended output wattage. It sounded great and I continue to use the pair today.

[Hons Gruber]

@Svein_B
Ok I have looked as this page do you think I should try to mod it the the 6550 tubes then?

@Alain Poitras
So the kt88/6550 is a btter option then the 807?

Wow didn't know the transformers were that expensive I knew they could get pricey, but not that pricey.
I don't know hat to about the speakers. I wonder how expensive it would be to send them to parts express and have them redone. They are around 40 years old now. The 10" have some small tears probably from being old. I didn't notice it when I bought the amp, but upon closer inspection at home... probably could have got it a little cheaper if I would have seen that... but I got it cheap enough.

So as far as testing it you think I should take a 1/4 cable stick in the "signal line out" and stick in the input of one of my other amps? I am no tube guru by any means in fact I know very little about them, but I have been learning about them or atleast trying to the past couple of days So if I plug it in a power it up if there are no tubes in it or one tube in it, it won't like start on fire or die or anything? I have not even plugged it in yet, because I know tube amps can have some crazy voltages and you need to make sure they are discharged before you work on them. Being that i am almost positive it has not been turned on for atleast 20-40 years I am guessing there is not a lot of dangerous voltages although I could be wrong.
Also thanks for the schematic that should be very helpful.
Well if the transformers cost a lot that is one more reason to change the tubes, because these tubes atleast from what I have heard can blow out the transformers sometimes or overload them or something not good...


@toprepairman yes those tubes I think overload some of the parts of the amp. another reason I want to replace them...

@6BG6GA
so theoretically I could just get some kt-88's? and I wouldn't need to rewire anything? just reset the bias? Is there a way to do that with out getting the bias master?

thanks guys hopefully I can figure this out with your help do you guys needs pics at all?
shalom,
Matthias

[Original Burnedfingers]

quote
Is there a way to do that with out getting the bias master

I would suggest a slight modification to the bias circuit. I would add a second bias adjustment pot so that individual bias adjustment could be made. Use a 10 ohm cathode resistor to ground on each output tube and adjust for 50mA per output tube.