Amp Tech Thread / Ask a tech Q

[dunwichamps]

I'm sitting here biasing a DSL100 and eating my lunch. The dual bias pots aren't help it. It is however doing a great job of making a set of JJ EL34 tubes unbalanced. If this was a JCM800 I would have been done with the amp before I finished my applesauce.

no experience but probs a crap amp then

[FullCustom]

I'm sitting here biasing a DSL100 and eating my lunch. The dual bias pots aren't help it. It is however doing a great job of making a set of JJ EL34 tubes unbalanced. If this was a JCM800 I would have been done with the amp before I finished my applesauce.

no experience but probs a crap amp then

Don't fear what you don't understand

[dunwichamps]

i dont fear it, but if you cant bias it with dual pots to within a mA or so then something else is problematic. either the tubes are bad or the bias pots dont have enough negative swing ect ect.

how are u measuring cathode current?

[FullCustom]

I can't tell the back line company that owns this amp that it's a crap amp. I have to fix it and it has to be stock when it's done. So far it is looking like the output transformer lead dress was the problem.

[dunwichamps]

I can't tell the back line company that owns this amp that it's a crap amp. I have to fix it and it has to be stock when it's done. So far it is looking like the output transformer lead dress was the problem.

was the OT taps shorting? Or are the leads too close to sensitive circuitry causing PO?

[FullCustom]

Marshall does this thing where they tie wrap all the OT leads to the back panel. It's neat and tidy but it's wrong.

[dunwichamps]

Marshall does this thing where they tie wrap all the OT leads to the back panel. It's neat and tidy but it's wrong.

ahh yes they dont do the split transformer setup (ala Hiwatt and others) which I think is a far superior way of doing the OT/PT positioning. Is there enough lead slack to rerun the wires are will you have to extend them to reposition?

[FullCustom]

Um, go look at some JCM2000 pic. The OT positioning is fine, it was just the lead dress.

[dunwichamps]

Um, go look at some JCM2000 pic. The OT positioning is fine, it was just the lead dress.

my bad. So they run the primary with the secondary along the back panel?

[FullCustom]

Um, go look at some JCM2000 pic. The OT positioning is fine, it was just the lead dress.

my bad. So they run the primary with the secondary along the back panel?

Yup, tie-wrapped together nice and neat like. The RF bleed was making one side run hot.

[dunwichamps]

Um, go look at some JCM2000 pic. The OT positioning is fine, it was just the lead dress.

my bad. So they run the primary with the secondary along the back panel?

Yup, tie-wrapped together nice and neat like. The RF bleed was making one side run hot.

ahh so their was coupling between the primary and secondary causing PO and excessive current. That sucks.

[FullCustom]

Happens on a lot of amps, Blue Jr's are famous for it. People blame it on the PCBs but it's just OT lead dress.

[dunwichamps]

yea i thought rule of thumb was keep your OT primary and secondary leads far away from each other, and the secondary far from the output tube sockets.

[FullCustom]

Yes, sir, correct.

[RAGER]

Man wouldn't that be amp building basics??.  Fender and Marshall have been making amps now for how long?  shit

[Mr. Foxen]

Have I mentioned Marshall are shit and gay?

[RAGER]

Have I mentioned Marshall are shit and gay?

If my Marshall is gay then it is one mean ass bull queer

[dunwichamps]

amp makers have long been cutting corners from underspecced PTs, improper heater wiring, lack of shielded cabling on sensitive grid wires, bare minimum spec resistors, underspecced caps.....

[FullCustom]

And the PV VK100 is the biggest turd of them all. You have to RF Snub the hell out of the OT to make it work.The only amp that I have ever see that is less stable with NFB.

I see shielded cable as more of a band-aid. I've seen shielded cable work more as an antenna than a ground plane in some amps(Dr Z/later Fender).

[dunwichamps]

you dont wanna overdue the shielded cable since often it has no benefit but the first grid wire from the input, and usually the NFB run from the Tap Selector back to the NFB resistor are the 2 most important shielded runs I do in my amps. The result of the grid wires are so short that I wouldnt see much benefit from shielding them and its more of a pain to run a coax cable there.

[bass sic]

Real quick guys, and then back to the discussion. I have two 50wt speakers that ohm out at 12 ohms. That would make them 16 right? And if I wire them series they come in at 22 ohm which is too high. Wired parellel they read 6 which would make it a 8 ohm cab. Am I right here?

[dunwichamps]

Real quick guys, and then back to the discussion. I have two 50wt speakers that ohm out at 12 ohms. That would make them 16 right? And if I wire them series they come in at 22 ohm which is too high. Wired parellel they read 6 which would make it a 8 ohm cab. Am I right here?

yea generally the DC resistance is usually a bit smaller than the impedance so thats a solid bet

[bass sic]

Sweet thanks. I'm putting the final touches on this cab build and hope to be test driving it tonight.

[liquidsmoke]

(Ampeg SS150 head, B(clean) channel volume is a lot more quiet than it used to be, the destructive event happened at practice weeks back) update:

Second shop said they can't find anything wrong with it. Fortunately I'm only out $36 so far. Decided not to try to work on it myself although I did look at the innards with a friend who knows more than I do(which is nothing) about electronics.

This amp used to be insanely loud, now it's only fairly loud. It's still usable but it sounded better before. First shop said it tested at 150 watts which is what the back of the amp listed. I'm confused. Having thoughts about trying to ship it to one of you amp builder types to do what is needed to get it working like it should. Or I'll just buy a solid state rack power amp and a ss rack pre amp that can do super clean tones. Or try to have one of you amp builder types rip the pre amp out of the Ampeg and put it into a stand alone box to be used with a power amp because I like how I can crank the bass knob and make my speakers fart even if I don't do that normally. It can do a Laney AOR like bass boost times 2. From what I've read some bass players actually use this head although I would think the power might be a bit on the low side for loud rock and metal.

[mortlock]

what would it take to replace rca jacks on an old synth..is it hard to do??