Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on November 28, 2017, 02:09:13 am
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Hi guys, Got another Bassking amp that has been pretty hacked up.
(https://i.imgur.com/3O19MKV.jpg)
(https://i.imgur.com/FFZX714.jpg)
Now that I have some info on the original circuit, it seems pointless to have two channels the same. :think1:
(https://i.imgur.com/1GPAekH.jpg)S
Since it has been hacked there is no real vintage value and my stocks of MUSTARD caps are running low. :sad2:
So I'm thinking I'll keep the BASS channel stock and tweak the normal channel.
(https://i.imgur.com/vdvNWM3.jpg)
Removed the unwanted components, and now have two spare socket holes (one was already vacant)
My thoughts on a second channel
(https://i.imgur.com/gGSSuwA.jpg)
But I'm just thinking now that a parallel 12AX7 in V3 might be the go as the hole the right size. :think1:
V1a also will need to be tweaked to suit, I'm thinking a "COLD KIPPER"
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A couple of ideas....
(https://i.imgur.com/bv69dkq.jpg)
(https://i.imgur.com/WM2yVrf.jpg)
Component values not finalized.
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Timbo,
I can NOT remember IF you have ever built DaGeezer's HoSo56 or not? It's absolutely one of the most fun touch sensitive amps I've built or
owned. And one of the few amps I regret selling.
Here are some ideas for you to consider .........................
IF you don't have a 5879 available for V1, you could use a 6AU6 or a 5654.
With respect, Tubenit
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Thanks T, I have just completed this Hammond organ amp conversion.
http://el34world.com/Forum/index.php?topic=17565.0
One of the local guys has presented this.
https://robrobinette.com/How_the_Marshall_JCM800_Works.htm
And I have used the "cold clipper" before and find it's a useful circuit.
So this is what I have.
(https://i.imgur.com/YnewpIk.jpg)
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Hi guys, Given this a way too much thought.
So a Trainwreck preamp is what I have decided on.
Simple circuit. :icon_biggrin:
(https://i.imgur.com/pAoDTV8.jpg)
Now, where to attach it.
I'm thinking a coupling cap and mix resistor connected to the other mix resistor (220k470k junction)
But I'm thinking the extra gain stage before the PI might make it TOOO gainy.
OR connect to pin2 V2.
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I think your 220/470 better. are you trying to stay "in-phase" with both channels?, if not you could drop V1a
otherwise;
I'm thinking the extra gain stage
that's why we use volume controls :icon_biggrin:
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Hey shooter, Yeah this is where I'm lacking in the technical stuff.
V1a I think is where the TW gets is mojo, so I guess i'll have to keep it in.
I'm a trial and error guy, so i'll just have to build it and see.
Being point 2 point makes it easy to mod.
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I'm lacking in the technical stuff.
:laugh: I was well trained, even made a good career from it, but I just wanted to climb rocks n drive jeeps so most of the technical went to sleep :icon_biggrin:
keeping the 2 channels phased allows you to play through both at the same time, musically, easy to remember; if the signal goes in G1 and out the plate it's flipped (out of phase), if it goes in G1 and out the cathode, it's the same (in-phase).
by the shear number of amps you've done, some of the techie has rubbed off :occasion14:
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The 22K-100r NFB around the output stage does about nothing.
Don't head-bang that now. After smoke-test, and gross voicing, pick your likely speaker system. Try making "22K" 10K, 5K, 2K, in there. Lower R here is lower gain and less nasty. It can depend a lot on the speaker you use. If you need "any speaker" tuning, find the value that is "too polite" and then we will add a pot to reduce that (dial-back some nasty).
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Thanks PRR, Changed the 22k to 2.2k for now, so far seems fine.
Injected life into this old girl and sounds great.
(https://i.imgur.com/7Dt0w1y.jpg)
(https://i.imgur.com/dZ7Hetx.jpg)
(https://i.imgur.com/iWUvE4n.jpg)
I've connected the TW preamp as per schematic and works well, TOOOO well, it's as loud as hell.
Might try connecting it to pin2 V2.
Will I need to use the 220k mixing resistor.
(https://i.imgur.com/yOyc1QT.jpg)
(https://i.imgur.com/62JHe9C.jpg)
(https://i.imgur.com/dyv7MN0.jpg)
(https://i.imgur.com/Ndu5UxA.jpg)
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Might try connecting it to pin2 V2.
Will I need to use the 220k mixing resistor.
You could try that. Or, you might replace that 150K on V1-2 with a 100K or 250K pot wired as a standard volume control and see if you can tame that wreck a bit. If you find a point that you like then measure pot resistance (wiper to each outer lug) and replace the pot with two resistors, or just leave the pot for more versatility.
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Thanks sluckey, I was looking through the TW schematics in the library and it was suggested that the 150k could be reduced down to 33k for this reason.
I'll give a pot a go.
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I'll give a pot a go.
Let us know how this turns out. I have an interest in this Trainwreck preamp.
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Are we talking about the 150k from V1a grid to ground? It looks like you'd lose all your bass (and midrange!) by reducing that to 33k, though a pot or other voltage divider would work fine.
You could also substitute a 12AU7 (or 12AT7 etc) to decrease gain leading up to the phase inverter. You'd want to tweak cathode bias resistor values if you went all the way to a 12AU7.
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Thanks guys, Added the pot as suggested and it did what you would expect, controlled the overall volume.
Replaced the pot with a 150k/75k combo (75k to ground) to get it to a usable volume and still not getting much rotation in the volume pot.
Paralleled the 1M grid leak resistor on V3b to tame the humbuckers and this helped a bit.
Checked the operation of the tone controls and they are working fine.
Reduced the 75k to 33k and removed the 150k and I think i'll stick with that.
I'll try some speaker rolling just for fun.
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Hi guys, What is speaker wattage should I be looking at???
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What is speaker wattage
If I did the math right you're <20W out, so 25-30W, maybe 40W if you want a stiffer sound :dontknow:
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Thanks shooter, most of the vintage speakers I have are rated at 25w and 15ohm.
I think I have some old eminence speakers but they might be 8ohm.
:icon_biggrin:
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Hi guys, This one is done.
(https://i.imgur.com/PexbZie.jpg)
(https://i.imgur.com/pN9yOyO.jpg)
(https://i.imgur.com/oJ9xziL.jpg)
(https://i.imgur.com/5FgfpYc.jpg)
The trainwreck circuit is great, lots of sparkle without to much hash.
There is a big volume difference the two channels, but I though this would be the case.
A jumper works well with the bass channel for some added tone and because there is a volume difference, the bass can be turned to max to added some drive at lower volume.
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Just found this ad:
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Hey PRR, Playmaster were huge back in the day.
As a kid I don't remember their stuff , I think "transistor" was the word.
I got one of those playmaster's on the shelf.
should dig it out for a look. :think1: