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Old 6th January 2013, 09:22 AM   #81
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Quote:
Originally Posted by cyclecamper View Post
Now I've got the first real problem with this amp. There's just one band of the rotary graphic EQ which does nothing at all. There's only 3 parts involved: a small inductor, a cap, and a rare pot with a strange fourth tap. So I'm going to hope it's the capacitor. With the cap and inductor in series either could cause complete failure. Of course, I should check the wires soldered onto the pot first!
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Old 6th January 2013, 09:25 AM   #82
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well stupid bloody bad wear stuffed that all up.
I meant to say
If anyone knows wear to get those 4 tag pots PLEASE let me know. as i need 2 to fix my Jansen Bassman.
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Old 6th January 2013, 10:37 AM   #83
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Did you ever get new tubes for it? and what did you decide on getting?
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Old 6th January 2013, 11:05 AM   #84
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Hi,

gorgeous project!

Some thoughts on it, and answers to some questions:

The SVT's final stage is not an Ultralinear design, just straight pentodes. You could modify your amp to get it switchalbe between both modes, and even a third one, the triode mode, by using a dual pole triple throw switch of sufficient voltage capability (!). Just connect all three screens of every push-pull side to both the wipers and the three fixed contacts to Eb (for pentode), or UL taps (for Ultralinear) or plates (for triode mode), respectively. Also the SVT doesn't use an interstage tranny, as far as I know.

I'd never replace 6L6GC's by 6550's ot KT88's, due to their bigger heater current (1.6 vs. 0.9 amps) requirements which most likely will burn the mains transformer's heater winding.

I'd ever replace any carbon resistor by it's modern, and reliable metal film equivalent. There might occur some sound change, due to the fact that you bring it back to it's factory sound when the original carbon rsistors still had their original values.

I'd also ever replace all electrolytics. Due to ageing their capacitance values are prone to decrease, while leakage current most likely will increase to unwanted amounts.

I'd insert 10 ohms resistors into the cathode lead of every final tube. Thus you can "match" them by your own to a sufficient level for (bass) guitar amplification.

Last, but not least: As you intend to use it as an bass guitar amp, I'd replace the five capacitors in series with the inductances in the equalization circuitry by some of four times the original value, thus bringing the equalization ranges an octave down.

Best regards and good luck!

Addendum: In real pentode mode your amp will deliver even more output power, but distortion will increase somewhat.

Last edited by Kay Pirinha; 6th January 2013 at 11:16 AM. Reason: addendum
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Old 7th January 2013, 12:47 AM   #85
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Kay, I can't make out exactly what you are saying. If I changed tube type to one that required more heater current its not a big deal to make a seperate heater supply just for the outputs. This amp is new enough to have modern caps. The main caps have been replaced. I do intend to do some work on the power supply filter ladder though, until an unfiltered scr light dimmer injecting line noise doesn't affect the amp (thru the 115 VAC; the RF noise from a dimmer radiated thru the air from the dimmed light lines is a seperate subject).

I may try pentode mode! But for now I have a lot of other things to address first.
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Old 7th January 2013, 12:56 AM   #86
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Indeed, the 'studio bass' version of the (almost) same circuit has the same inductors but bigger caps. That does move the frequency cneters down, but it seems to me that would also change the 'q' and make broader bands (which might or might not be a good thing).
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Old 7th January 2013, 04:18 PM   #87
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Hi,

yes, if you'd consider replacing yout output tubes by 6550's, you'd need an additional heater supply. Also don't forget that both tubes require different plate-to-plate load impedances.

As they were mentioned above: EL34's are not a good substitution! The also require more heater current (1.5 A vs. 0.9 A) than 6L6GC's and screen voltage in this UL circuitry most likely will exceed the maximum rating given for these tubes. Unless you can buy a sextett of NOS tubes (from Mullard, Telefunken etc.) for a huge amount of money, as these really are capable of the higher screen voltage, you'd better forget that idea.

You could double both the inductors' and the capacitors' values within the EQ circuitry, Q then should remain the same. But I guess that you don't even know the inductors' values, so changing the caps should be the best, and maybe the only possible, way to start with. Finally, listening experience tells you whether you're on the right way.

Best regards!
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Old 7th January 2013, 05:59 PM   #88
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I bought another 6L6. It's a JJ and not identical to either type I have now. I'm planning to convert to bias pots ASAP. Anyway, it doesn't get as hot now and works great. So I'm sticking to 6L6GC for now, while I address more pressing issues. I really want to move that 'distortion' stage to become another switchable gain stage w/ volume, located between the tone circuits and the EQ circuits.

Then I've got other heads to reassemble! And I should have at least one real bass bottom with with some larger low-frequency speaker in a bigger box, as alternative to all these open-back 4x10 and closed-back 4x12.
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Old 12th January 2013, 02:04 AM   #89
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teee Heee... rebuilt My '64 Fender Bassman, a few years back. Replaced ALL carbon comps with 1% carbon films, fitted all fresh caps Sprauge Atoms etc. etc.
Oem resistors had seriously drifted. Oem caps were either leaking already or close to it.
Utah speakers were "fine' as were the OEM Sylvania's
Sound.. was... different as result of it being back to factory circuit spec...clearer, brighter with far less brownout, notably louder too.
Took about a week to get used to the as new sounds. However all who have since listened / used it prefer the restored 'as Leo intended' voicing.
The Weenies can moan about it's not sounding Clapped out vintage.
Ohhh but wait.. It doesn't belong to them :-)

Last edited by Bare; 12th January 2013 at 02:07 AM.
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Old 13th January 2013, 03:18 AM   #90
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Well, this sure isn't a bluesy saggging-power distorting amp wheezing out an emotive cry. It's not a primary guitar amp, it's a clean machine primarily for bass and occasional guitar duty, though it might have more tone potential later if I add switches for non-ultralinear mode, or to remove some feedback. And of course if I move that extra pre-amp stage to between the "tone" section and the "EQ".

So I picked up a huge inductor in an attempt to get rid of every bit of power amp hum, yet not drop B+ much in a retrofit application. It's truly huge and really heavy, so I'm investigating many mounting alternatives. I should run over there and measure the DC resistance, as I bought it without knowing, just figuring based on its size that it must be low. And with the big inductor and more caps I will need to upgrade the rectifier or it will blow at turn-on. Turn-on is going to stress the transformer more than it's ever been pushed before, so I'm not going overboard with the size of the caps.

Now...it models with steady load, but should I be thinking about dong anything to dampen any resonances in the power supply due to the capacitance and inductance I'm adding?
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