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Ampeg fliptop

11/26/2005 6:49 PM
Floyd Ampeg fliptop
Just finished going through an SB-12 I got  
free from my bro-inlaw (works at a whse),  
whoo hoo!  
I changed the filter caps all except the  
30@ 600 and adjusted the speaker connections,  
also put in a grounded cord and unhooked the  
death cap, looks like it has all the original  
tubes, I get a frying eggs sound but the volume  
is all there and it sounds good but for the  
crackley sound, I have never had power tubes  
go out on me before so I don't know what the  
symptoms would be, when played loud I can  
see both 7868 tubes light up brighter by a  
good amount each time a note sounds out,  
never whitnessed that before so it does not  
seem normal to me. I wouldn't be suprised  
if the tubes were bad but I just wondered if  
anyone ever had this happen. I am not  
sure since the power cord is grounded now  
if I should also remove the .5uf cap  
seperating the 6.3V center tap from ground?  
How do the new 7868 tubes stack up against  
vintage? Anyone think it's worth the extra  
cash to go NOS or are the new tubes sounding  
Though I have not played it long I can tell  
the breakup souds really good when dimed and  
I know I will love the amp once I get it to  
quiet down. How do I get 30uf 600V from  
off the shelf stock? I don't want to put in  
a larger than stock value as I really like  
the way it sounds now.  
11/26/2005 7:02 PM
Don Symes

You might want to go back over the bias network and check the bias current - it sounds like the bias supply might be sagging (due to aged caps). That or the screen connections are going.  
Have you tried R.G.'s Tube Amp Debugging site?
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11/26/2005 7:59 PM
Thanks Don, I would like to check the bias but don't  
have any 1 ohm resistors handy, I was thinking  
of trying the Output Transformer Shunt Method  
but I don't know if my DMM Ammeter has a low enough  
series impedance to be accurate, I have an HP 3435A,  
I am not a tech and have built way more  
amps than I have fixed, most of which worked  
right out of the box so I don't have a great  
knowledge of troubleshooting...yet.  
The only grid resistors on the amp are the grid stoppers, i am thinking you may be right on the money with the bias sag as the tubes are "pumping"  
when pushed, i am going to try to measure the bias  
but am worried that if my meter is the wrong  
type I may do some dammage trying to set the  
11/27/2005 10:54 AM
Don Symes

[QUOTE]i am going to try to measure the bias  
but am worried that if my meter is the wrong  
type I may do some dammage trying to set the  
Set this one aside until you have some 1-ohm resistors handy, then. No sense blowing up a nice classic with rare tubes out of simple impatience.
11/27/2005 7:18 PM
John Martin
Don't disconnect that .5 cap on the PT center tap or you will have lots more hum from the heaters. In fact, I usually replace it in old amps with a lot of playing time.  
I just put in a new set of EH 7868 tubes in a Gemini I and my customer was very pleased. They sound good so don't be afraid to try a set.  
There is a 33uf/630 volt polypropalene Solen cap I use for Ampegs that have that 30/600v electrolytic. The Solen's width is much larger than the original electrolytic and I know it fits in B15 amps. If my B12 was nearby I would open it and look, but I believe I put one in it also. Due to its size, I wrap it in two layers of "fish paper" so the outer wrapping of the cap doesn't get punctured. Fish paper is a heavy duty paper used for electrical insulation. I think I got mine from Antique Electronic Supply. They also carry the Solen caps. A plastic "ty wrap" with a screw hole in one end will fasten the cap to the original mounting.  
11/28/2005 2:29 AM
John, is this a good way to lessen 6.3V hum?  
Never seen that one before.  
I hope the old tubes in there now are good for awhile so I can compare them to new production  
I thought there was a trick to using the poly  
caps for filters involving some resistance values  
in the PS, I remember someone over at  
Antique Electronic Supply told me filter supply  
caps need to be electrolytic and would not  
sell me poly caps as a sub, then there is  
the leakage current balancing trick when  
joining caps in series which I know nothing about.  
Couldn't be as simple as joining two 16uf 400V  
electrolytics in series, right?  
Thank you so much for your help guys.
11/28/2005 8:30 AM
John Martin
There is no "trick" to using the poly caps for filters. I have been doing this for years on both repairs and custom amps I build. The only problem is the size of the cap for replacment uses. They are superior to electrolytics. With a 600v cap you don't have to worry about the equalizing resistors.  
Two 16uf caps in series will give you 8uf.  
I have no way of knowing what condition your power tubes are in. What I do know, from servicing hundreds of Ampeg amps, is that people tend to leave the old tubes in them until the amp goes into melt down. If there is any question of the tubes condition, replace them. Its not worth losing an output transformer, etc.  
There is better advice here on Ampage then can be gotten from the parts distributors however well meaning they may be.  
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