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
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
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
Circuit diagram for a QS with 4817: 8417 Amps
Conversion to 6550: http://www.triodeel.com/8417.htm
SB
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.
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.
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 ...
An externally hosted image should be here but it was not working when we last tested it.
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.
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. 🙂
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....
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.
@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
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
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.
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.
That's for the "8417 amps" schematic link in the Svein_B but the Hons Gruber amplifier schematic is the big one in my post ... The B+ is 600V but the bias can be adjust for the maximum current.
In the 6550 Tung-Sol datasheet, they said the maximum plate dissipation is 42W but in the KT88 Genalex which is a similar tube, 42W is an absolute maximum and the design maximum is 35W ... But in both datasheet, for a 100W output, class AB1 fixed bias, the plate dissipation is less than 35W ...
An externally hosted image should be here but it was not working when we last tested it.
I suppose the output transformer for the 8417 in the Hons Gruber amplifier is a 4200 ohms PP but it should be OK for 6550 or KT88 tubes with a 600V supply.
In those "typical operation", the plate dissipation with no signal is 33,12W for the KT88 and 30W for the 6550. The fixed bias are respectively -34V and -32,5V, so Hons Gruber will have to modify a little bit the bias adjusment of is amplifier to use a 6550 or a KT88 because it range about from -11V to -24V ... The pinout is the same as the 8417.
The bias circuit can go up to -35V so just bypassing the R60 22K resistor will do the job. But if he use a KT88 like in Genalex typical operation, he should provide a separate bias adjust for each tube ... If ever -35V is too low, he can add a small value cathode bias resistor and bypass capacitor to each tube to raise the bias a little bit higher.
Alain. 🙂
Like I just write in my preceding post, he will have to modify the bias circuit to get up to -35V, that's the only modification he have to do.
By the way, I look at your pseudonym, 6BG6GA, I didn't know that tube up to now, I look at it in my databooks and it is a very interesting one, it seem to be good for screen grid drive too ... And they are not very expensive on eBay ...
Alain. 🙂
If your speakers just have little tears, maybe you can repair them with a some glue or silicon, a friend of mine use to repair that with a little piece of "elastoplast" at the back of the cone, it stick very well to it and is flexible.
Having the amplifier working with no tubes will cause no problems, if you look at the schematic, the only commun part of the power amp and preamp is the power transformer, it's almost like two completely separated amplifier. The tubes power supply is a half wave voltage doubler and there is some resistor across the filter capacitors, so when you turn the power off, they discharge very fast through those resistors and there is no more charge in them. But when you take some measure in a circuit like that with the power on, 600V is a very dangerous voltage that can kill you, take care.
Have a look at the big filter capacitors, if they leak or look "hunched", they may have to be replaced, electrolytic capacitors are "perishable". If they are no good anymore, the power amplifier will produce a very big 120Hz hum ...
You can use some KT88 instead of 6550 because those tubes are similar but they are not the same type, you better use the 6550 because the KT88 need a separate bias adjusment for each tube.
Like I said in my other posts, you will have to bypass the R60 22k resistor to get the correct bias because it must be much higher than for the 8417. A good trick to adjust the bias is to measure the DC resistance of each side of the primary transformer with the power off and after, with power on, starting with the higher bias , about -35V, measuring the voltage drop across those winding with no signal for the right plate current.
The maximum plate dissipation of the 6550 is 35W, with a 600V supply, the current on each 6550 must not exceed 58ma because 35 / 600 = 0,058A ... Suppose the measured DC resistance of each side of the primary is 200 ohms, the bias must be adjust to get 11,6V because 200 x 0,058 = 11,6V ... But like I said before, this is very dangerous high voltage for you, you have to know exactely what you are doing.
If you just put the 6550 on without modify the bias circuit to get up to -35V and adjust it carefully, you will fried your new 6550 tubes because the bias of this amp is actually less than -24V ...
With the power off, you can verify with a ohmmeter if the winding of both transformers are OK, not short and not open, you can also check most of the parts and the wiring with your multimeter but of course, you have to know the electronic basis and how to read a schematic ...
However, the first step is really to try the preamp using the "signal line out" plug to a sound system, if it don't work well, you should repair it before spending money for new tubes. If you don't know much about electronic, you should have a friend to help you because testing and measuring parts and circuits like that need good electronic knowledges.
Alain. 🙂
Hey guys sorry it took so long to get back. been workin' a good amount as of late.
Well I learned a valuable lesson about tube amplifiers tonight never and I mean never work on it while on with wet hands....😀😀😀 ok I didn't do that... haha
😉
However I did test it and besides some scratchy potentimeters the preamp area works fine. In fact the voicing section it has is pretty cool. It has some modification stuff to modulate some old fender amps and add distortion. I got to say I am impressed with the inboard distortion especially with the time period and that its solid state. I have a solid state guitar amp from the 80's and it sounds great, but the in board distortion is absolutely hideous thats what pedals are for though....
So whats the best way to fix scratchy potentimeters? Is there a way to clean them? Also the inputs or atleast one of them makes some crackling sounds when inserting or withdrawing the quart inch. I haven't seen input like these you can see through them, since they have no back.
So basically for the tubes all I have to do is change the bias? And I will do this by bypassing the r60 22k resister.
I believe I have an dc tester and ohmmeter around the house somewhere if not I am ordering one.
Also for tubes would the Electro-Harmonix 6550 Matched Power Tubes be pretty good? I also have a question about them that maybe you can answer what does it mean when you can order them soft. medium, or hard?
Thanks for all the help it is all starting to make sense. I have wanted to be good with electronics wiring and to have all of that knowledge. I will probably never know as much as you guys, but you guys are helping me to understand. Someday I think it would be cool to build amps. Wish I had an electronics class in school
but well that would be intresting you know and we can't have that 😀
Well I am not sure i have any close friends that know electronics inside and out, but I will learn about this stuff before I do it that is for sure. I sure don't want to electrocute myself doing this or ruin the amp.
shalom,
Matthias
Well I learned a valuable lesson about tube amplifiers tonight never and I mean never work on it while on with wet hands....😀😀😀 ok I didn't do that... haha
😉
However I did test it and besides some scratchy potentimeters the preamp area works fine. In fact the voicing section it has is pretty cool. It has some modification stuff to modulate some old fender amps and add distortion. I got to say I am impressed with the inboard distortion especially with the time period and that its solid state. I have a solid state guitar amp from the 80's and it sounds great, but the in board distortion is absolutely hideous thats what pedals are for though....
So whats the best way to fix scratchy potentimeters? Is there a way to clean them? Also the inputs or atleast one of them makes some crackling sounds when inserting or withdrawing the quart inch. I haven't seen input like these you can see through them, since they have no back.
So basically for the tubes all I have to do is change the bias? And I will do this by bypassing the r60 22k resister.
I believe I have an dc tester and ohmmeter around the house somewhere if not I am ordering one.
Also for tubes would the Electro-Harmonix 6550 Matched Power Tubes be pretty good? I also have a question about them that maybe you can answer what does it mean when you can order them soft. medium, or hard?
Thanks for all the help it is all starting to make sense. I have wanted to be good with electronics wiring and to have all of that knowledge. I will probably never know as much as you guys, but you guys are helping me to understand. Someday I think it would be cool to build amps. Wish I had an electronics class in school
but well that would be intresting you know and we can't have that 😀Well I am not sure i have any close friends that know electronics inside and out, but I will learn about this stuff before I do it that is for sure. I sure don't want to electrocute myself doing this or ruin the amp.
shalom,
Matthias
That's a very good news, the preamp is a big part of the amplifier !... However I did test it and besides some scratchy potentimeters the preamp area works fine ...
... So whats the best way to fix scratchy potentimeters? Is there a way to clean them? Also the inputs or atleast one of them makes some crackling sounds when inserting or withdrawing the quart inch ...
... I believe I have an dc tester and ohmmeter around the house somewhere if not I am ordering one ...
... Also for tubes would the Electro-Harmonix 6550 Matched Power Tubes be pretty good? I also have a question about them that maybe you can answer what does it mean when you can order them soft. medium, or hard?
... Wish I had an electronics class in school
Sometime, the scratchy potentiometers can be clean with a special cleaner in spray, you put some inside (with power off) and turn the knobs back and forth hundreds times ... Let it dry and try them ... If they are still scratchy, do it again ... But in the worst cases, there is nothing to do and you have to replace the potentiometers, unfortunately, the same models can be very hard to find !
Here's two brands of those spray cleaners :
Asalco Cleaner and Lubrican - Deoxit Faderlube
The Asalco Cleaner and Lubrican for contact can do and is cheap ...
The Deoxit Faderlube is the best potentiometers cleaner in the world but is more expensive ...
If some input jacks are broken, they are very easy to replace but if they work well even if they make a "crackling sounds", it's not really a problem ...
About soft, medium and hard :
Those are tubes matching ranges, that's the current versus the bias voltage ... If "hard" tubes was install on the amplifier and you replace them with other "hard" tubes, you don't absolutely need to readjust the grid bias for them ... But if you replace them with "soft" tubes without reajusting the bias, the current will be too high and the plates will become "hot red" and the new tubes will not last very long ...
The bias voltage must be higher to get the same current with "soft" tubes than with "hard" tubes, since in your amplifier, you will have a maximum bias voltage of only -35V, you should order "hard" tubes to make sure to be able to adjust the bias to get no more than 50ma current for each tube, the 6550 plate dissipation will be about 30W.
Here's a thread about that on Telecaster.com
If you have a digital multimeter or buy one, make sure the maximum DC voltage scale is 1000V because you will have to measure over 600 VDC in this amplifier, but like I said before, this is very dangerous for you ...
Before you buy your new tubes, you should verify if the tubes power supply and both transformers are OK. You can verify that with no tubes on with your multimeter diretly on the power tubes socket on top of the frame, there is no need to work inside.
I make a little drawing for you, it show where to put the multimeter probes to measure those voltages, you have to do it on both sockets, if the output transformer is good, the plate 600V will reach those sockets on pin 3 :
An externally hosted image should be here but it was not working when we last tested it.
Notice the tubes pinout on the datasheets are alway "bottom view" with pin numbers "clockwise", that's why on this drawing the socket "top view" is numbered "counterclockwise" ...
When you measure the voltages on the sockets holes, don't put the probes deep in them because there are parts under and you can accidentally make a short circuit ...
The cathodes of the tubes on pin 8 are connected to ground, so you can put the negative probe (black) of the multimeter on this socket hole when measuring plate, G1 and G2 DC voltages ... The heaters voltage is AC and must be measure at number 2 and 7 socket holes.
All the voltages will be higher than on my drawing because there is no tubes, then no loads ... You will get over 600 VDC for the plates, over 300 VDC for the screen grids (G2) and over 6,3 VAC for the heaters. If you get lower voltages than on my drawing, there, is a problem ...
The bias voltage for the grid (G1) depend on the actual bias potentiometer setting, this is the 25K TR-3 trimpot you will have to adjust for your new tubes bias. But it should be OK because it come from the same supply than for the preamp and this one is working well ...
When you bypass R60 22K (red red orange), you will be able to adjust the bias from about -17 to -35 VDC, you don't have to remove this resistor but just solder a small wire across it for bypassing ... The best will be adding another trimpot and resistor to be able to adjust both tubes separately in case they are not perfectly matched, this can be a future improvement ...
You never have electronic courses at school ...
There is no need for that, you can learn all that on Internet, electronic books and magazines ... I have only a basic "electrotechnic" course in secondary IV when I was young in the early '70 and learn everything else by myself, I am a "self-made" electronic technician and work for electronic company almost all my life ... I study electronic since 40 years and always learn more things ... Anyway, today electronic courses said nothing about tubes, most young technicians and engineers have no ideas how a tube work, that is something you have to learn by yourself !
Give me some news about all that and feel free to ask me if you have any questions, helping other DIYers in their projets is a good way for me to alway learn more electronic stuff.
Alain.
QUIZ for tubes experts ...
I was reproducing the power amp schematic section of the Hons Gruber SG-100 guitar amplifier in my SIMetrix simulator to see what append when replacing the 8417 with 6550 and I found two big connexion errors on it ...
Guess what they are !
I will give the quiz solution later this week if nobody found it ...
Alain. 😉
I was reproducing the power amp schematic section of the Hons Gruber SG-100 guitar amplifier in my SIMetrix simulator to see what append when replacing the 8417 with 6550 and I found two big connexion errors on it ...
Guess what they are !
An externally hosted image should be here but it was not working when we last tested it.
I will give the quiz solution later this week if nobody found it ...
Alain. 😉
Yeah, that's the cathode resistor of the cathodyne phase splitter; should be positioned between R91 and R92 IMHO.
is it something to do with one tube having internal getter/catode connection, and the other not ?
mind you, I do not know these these tubes
other than its a classic error, right ?
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