Help With Two Rock OT Swap

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Thewayfarer
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Help With Two Rock OT Swap

Post by Thewayfarer »

So I have this great Two Rock Crystal (basically the JM circuit) that’s been built with NOS components. I’m going to swap the Classic Tone OT with a 1970 Twin Reverb OT. Since it’s 4ohm, I know I’ll be moving the NFB tap and lowering the resitor to 8.2k (for a starting point). Any other modifications that I need to make? Thank you.
Last edited by Thewayfarer on Tue Mar 06, 2018 11:57 am, edited 2 times in total.
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martin manning
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Re: Help With Two Rock OT Swap

Post by martin manning »

Sounds good. What's the value of the current feedback resistor, and what speaker output tap is it connected to? 12k to 8R?
Thewayfarer
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Re: Help With Two Rock OT Swap

Post by Thewayfarer »

Yes, 12k and 8ohm tap.

Dawning I’m me that my 1X12 will be rendered useless. Don’t know many 100W 4ohm speakers. lol.
mojotom
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Re: Help With Two Rock OT Swap

Post by mojotom »

I really like Bassman/twin NOS output transformers on those amps, but never tried to build a JM sig.

On the ODS and ODSR type amps (as well as amps like old Vibroverb) I prefer to mismatch at the output (running 4R tap to an 8R speaker) seems fatter sounding to me.
Still not a JM sig but I like to pair that OT (or the Bassman) with an 8R speaker.

You should try it.

Am I wrong thinking you'll need a bigger fdb resistor switching from 8 to 4R ?
Marshall used 27k@16R, 47k@8R and 100k@4R, didn't they ?
(I might be wrong on that point)
Last edited by mojotom on Sat Feb 10, 2018 10:03 pm, edited 2 times in total.
Bombacaototal
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Re: Help With Two Rock OT Swap

Post by Bombacaototal »

mojotom wrote: Sat Feb 10, 2018 5:42 pm I really like Bassman/twin NOS output transformers on those amps, but never tried to build a JM dig.

On the ODS and ODSR type amps (as well as amps like old Vibroverb) I prefer to mismatch at the output (running 4R tap to an 8R speaker) seems fatter sounding to me.
Still not a JM sig but I like to pair that OT (or the Bassman) with an 8R speaker.

You should try it.

Am I wrong thinking you'll need a bigger fdb resistor switching from 8 to 4R ?
Marshall used 27k@16R, 47k@8R and 100k@4R, didn't they ?
The primary of a Bassman OT is 4.2K. By mismatching as you described you are making it 8.4K. I am not sure this is good for 6L6?

As far as the GNFB to keep the same gain reduction one needs to reduce it
mojotom
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Re: Help With Two Rock OT Swap

Post by mojotom »

Bombacaototal wrote: Sat Feb 10, 2018 5:53 pm
mojotom wrote: Sat Feb 10, 2018 5:42 pm I really like Bassman/twin NOS output transformers on those amps, but never tried to build a JM dig.

On the ODS and ODSR type amps (as well as amps like old Vibroverb) I prefer to mismatch at the output (running 4R tap to an 8R speaker) seems fatter sounding to me.
Still not a JM sig but I like to pair that OT (or the Bassman) with an 8R speaker.

You should try it.

Am I wrong thinking you'll need a bigger fdb resistor switching from 8 to 4R ?
Marshall used 27k@16R, 47k@8R and 100k@4R, didn't they ?
The primary of a Bassman OT is 4.2K. By mismatching as you described you are making it 8.4K. I am not sure this is good for 6L6?

As far as the GNFB to keep the same gain reduction one needs to reduce it

Most of the 70's Dumble were built with a 4R tap and running an Altec 8R. I thinks that was he's first mod as Altec were only available at 8R.

I've been running mine for 10 years (with new and NOS tubes) and don't find any issue running it.
But I'm not an expert ;)
Thewayfarer
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Re: Help With Two Rock OT Swap

Post by Thewayfarer »

Well that’s encouraging. I didn’t really want to have to buy two new 8ohm 12-65s.
Bombacaototal
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Re: Help With Two Rock OT Swap

Post by Bombacaototal »

mojotom wrote: Sat Feb 10, 2018 7:59 pm
Bombacaototal wrote: Sat Feb 10, 2018 5:53 pm
mojotom wrote: Sat Feb 10, 2018 5:42 pm I really like Bassman/twin NOS output transformers on those amps, but never tried to build a JM dig.

On the ODS and ODSR type amps (as well as amps like old Vibroverb) I prefer to mismatch at the output (running 4R tap to an 8R speaker) seems fatter sounding to me.
Still not a JM sig but I like to pair that OT (or the Bassman) with an 8R speaker.

You should try it.

Am I wrong thinking you'll need a bigger fdb resistor switching from 8 to 4R ?
Marshall used 27k@16R, 47k@8R and 100k@4R, didn't they ?
The primary of a Bassman OT is 4.2K. By mismatching as you described you are making it 8.4K. I am not sure this is good for 6L6?

As far as the GNFB to keep the same gain reduction one needs to reduce it

Most of the 70's Dumble were built with a 4R tap and running an Altec 8R. I thinks that was he's first mod as Altec were only available at 8R.

I've been running mine for 10 years (with new and NOS tubes) and don't find any issue running it.
But I'm not an expert ;)
The Mayer amp is very much a 80’s circuit which I do not recall being mismatched while the 70’s amps are a different amp all together

In my personal opinion I am not sure it’s such a good idea
mojotom
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Re: Help With Two Rock OT Swap

Post by mojotom »

I could be wrong, maybe Max could help, I haven't been there for years but from what I remember most ODS up to 12# either combo or head with 1x12 cabs were 4R OT (either Bassman or Twin) running 8R speakers.

As for my personal opinion, for now, I favor this mismatch, either 50 or 100w. I did a few listening tests back then with a 70's, a low plate skyline and a high plate skyline and I like the added compression.

If I would swap a modern tranny by an NOS Fender Twin I would at least trying the mismatch and check how it feels and go from there.
Thewayfarer
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Re: Help With Two Rock OT Swap

Post by Thewayfarer »

This all came about because another Crystal owner had Bill K swap a 70’s twin OT’s into his (along with a slew of other NOS mods) and he Couldn’t believe how good it sounded when he got it back. I’ve About maxed out the board for NOS components, so I figured I’d to the OT as well (not worried about the PT). I believe he’s running a 4ohm load

I’ll try the mismatch, but it’s not exactly a hard fix as long as I’m using a 2x12. 1x12 (EV style) have to be mismatched.
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norburybrook
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Re: Help With Two Rock OT Swap

Post by norburybrook »

I Built mine with the heart of a 1980's Peavey Triumph 100w and it sounds great, and has 4.8 and 16 ohm taps. As I was building from scratch the changes to the rectifier/power board were an easy thing to do. just required full wave bridge rectifier and a voltage doubler for the BIOS.

Keep an eye out for the old Peavey USA amps they're great donors. Dumble built a couple of ODS in the #170 range from peavey iron.



M

p.s. I've run my fender bandmaster(6L6) which only has a4 ohm tap on 8 ohm speakers for 20 years no problem
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martin manning
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Re: Help With Two Rock OT Swap

Post by martin manning »

norburybrook wrote: Sun Feb 11, 2018 10:33 am...and a voltage doubler for the BIOS.
BIOS? Wow, there's an acronym I haven't seen in a while!
mojotom wrote: Sat Feb 10, 2018 5:42 pmAm I wrong thinking you'll need a bigger fdb resistor switching from 8 to 4R ?
Marshall used 27k@16R, 47k@8R and 100k@4R, didn't they ? (I might be wrong on that point)
To preserve the same ratio the FB resistor must be reduced. The voltage on the 4R is less than on the 8R.
Thewayfarer wrote: Sat Feb 10, 2018 3:16 pmI’m going to swap the Classic Tone OT with a 1970 Twin Reverb OT.
One caution: connect the Twin OT's primary leads in a temporary fashion unless or until you determine that you have the phasing correct
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norburybrook
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Re: Help With Two Rock OT Swap

Post by norburybrook »

LOL.........BIOS

ha that's my life of working with computers coming through....:D

it's Sunday morning here :D


BIAS!!!

:D

good point about the OT phasing. I suggest wearing ear plugs or defenders when first firing up!

M
Thewayfarer
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Re: Help With Two Rock OT Swap

Post by Thewayfarer »

Good tip. I usually have the dog hanging out with me too. I’ll clear her out!
Thewayfarer
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Re: Help With Two Rock OT Swap

Post by Thewayfarer »

No go. The Twin OT would have to be mounted 90s from what it should be and I’m not set up (nor really desire) to drill a chassis. Oh well...disappointing.
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