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Dirty-South Blues Harp forum: wail on! >
bassman 10 -> self split 5F special ??
bassman 10 -> self split 5F special ??
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bonedog569
639 posts
Sep 15, 2012
8:34 PM
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to un-hijack the 'one video, superb harp' thread, - I'll carry on here.
I've been sitting on a working silverface bassman 10 chassi I picked up for next to nothing a few years back. I converted it to cathode bias - but it still hasn't inpired me much. I was thinking of returning it to stock , building a head cab for it - then selling it . . . until I heard Jon blowing through Mark's (5F) self split amp. That thing sounds nasty good , - of course there's a nasty good player on the video we heard, - but never mind that.
I know it's more chassi than the self split needs - but what the heck - it's here ready to be Franken-amped. Will these transformers work for the purpose?
5F - you said "I would still do the same but split the preamp tube one triode each for V1 & V" Is this now your preffered disign for a simple amp without a phase inverter valve? Why is is better?
Is there any other Franken-amp over the above options you think are worth considering?
I've been 'jonesing' for the smell of solder.
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5F6H
1361 posts
Sep 16, 2012
2:21 AM
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Hi Bonedog,
The transformers will be fine, they're not under any more stress in self split that they would be normally. However, for best stage volume I'd check out whether the power transformer has enough juice for a pair of 6550 power tubes (they'll need 1K 5W screen grid resistors installing), does the PT have a serial number? They usually will take it, if they only use a couple of preamp tubes, best to be sure.
You might well be fine with the preamp tube layout as it is, I'd tweak the phase inverter for more of a black/brown face style, use a 12AX7 here...convert the master vol to a 6G12A presence pot? With the little amps only having one preamp tube, you have space to wire in a second to give more flexibility...you might not need to do this, then again it depends on how far you want to go - ideal situation would be 2 parallel but discrete channels, each tube being half a triode each for one channel - V1 is the input stage for both, V2 is tone stack recovery for both...
Cathode fed self split tends to like a lot of mids, on amps that have mid & presence, I usually have these full up & mid pot is 25K not 10K.
Here's a diagram for fixed bias to cathode fed self split switch, theoretically can be applied to any fixed bias amp...
For absolute best volume there is another way to go self split, by tapping the screen grid, this allows you to run fixed bias & more power than cathode fed...tone is not as sweet as the cathode fed system though, but no worse than regular fixed bias PI driven.
...the inversion is care of the 0.1 cap running from the in-phase power tube's screen grid (pin 4) to the out-of-phase tube's control grid (pin 1 on a Fender). Balance drifts at different volume settings and there is a big thump when switching from play to standby.
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Last Edited by on Sep 18, 2012 6:08 AM
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bonedog569
640 posts
Sep 17, 2012
8:04 PM
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Thanks Mark , I have to study the schematics (make some line drawings of the a and b switch circuits) and get back to you with questions when I get a chance.
Do you have any schematics up of a design with two discrete channels? -I 'll try lining one out myself in the meanwhile.
The PT serial - as best I can read it is J02 708 CSATESTED 6-2-09
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Joe_L
2058 posts
Sep 17, 2012
10:08 PM
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Ah, the Bassman 10! A friend of mine has one! We call it the Tower of Power. Its huge and heavy, but it has got to be one of the best deals per pound out there. Its not a bad harp amp.
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5F6H
1369 posts
Sep 18, 2012
2:56 AM
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@ Bonedog "Do you have any schematics up of a design with two discrete channels? -I 'll try lining one out myself in the meanwhile."
I'd have to draw one...but if you look at the Fender Concert 6G12 (not the "-A" version) preamp you'll get the idea (just ignore the vibrato circuitry). Rather than having V1 as 2 stages for 1 channel, you make it 1 stage each, for two separate channels (I don't usually bother with 2 channels). In fact if I was going to the trouble of rewiring the preamp, the 6G12 is the preamp that I would copy, just use a 25K mid pot/fixed resistor instead of a 10K if going cathode-fed self-split.
To give the self-split principle a go, you could stick with the current preamp wiring (there have been a few bassmans converted, just with the switch outlined above, no other changes)? I'd just tweak the PI for 12AX7 use.
That PT no. doesn't scan? Is that stamped on the end bell? I'd be looking for a no. like 125P5D (which is the same as a Super Reverb tranny which has a 5A heater winding and will support 6550 power tubes with 2, or 3 12A#7s).
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