amp gurus: how mod-friendly is the 70W ultralinear bandmaster circuit?

[dr. ad]

please forgive the difficult questions and the noob-ish nature of this thread.

i've owned a supertwin before, so i can't be talked out of categorically liking these horrible CBS circuits;
what can my more learned and experienced forum brothers tell me about these?

a glance at the schematic, and a vague understanding of these circuits' inclusion of transformers in the pre-amp stage leaves me wondering about the difficulty of possible mods, ie:

switching taps or entire OT so i can run it on our 240V(ish) mains

giving it a pair of KT66s (just a re-bias..?)

trying 12AT7 and 12AY7s in certain spots in the preamp

and how difficult it would be in theory to change the percentage of the primary transformer that the preamp uses?


i know this could be a tough one; full appreciation to all responses (even if it's just bumpage).

what say you?

[Stratman]

Re: amp gurus: how mod-friendly is the 70W ultralinear bandmaster circuit?

Suggest changing the power, output transformers, as well as the choke.

For the KT-66 tubes you will need to make sure that the power transformer can handle the higher amperage draw that the heater circuit used (e.g., 1.3 for KT-66 verses 0.9 for 6L6)

Finally, you will need to change the resistors for the KT-66.

Cost around 450 USD.

Tone change = Priceless!

[dr. ad]

Re: amp gurus: how mod-friendly is the 70W ultralinear bandmaster circuit?

sounds like you've been down this path; i'll take that on-board.

thanks

[Chickenwings]

Re: amp gurus: how mod-friendly is the 70W ultralinear bandmaster circuit?

Ad....there is some info you can glean from the torres engineering website on those ultralinears too...

[MikeM]

Re: amp gurus: how mod-friendly is the 70W ultralinear bandmaster circuit?

Do you want a UL amp? What kind of tone are you looking for?

Stratman is right. Best bet would be changing all the iron..

[dr. ad]

Re: amp gurus: how mod-friendly is the 70W ultralinear bandmaster circuit?

^ well, i've owned one before (180W super twin) and although it definitely lacked the sweetness of a lovingly maintained original blackface, i've never heard any other valve amp with the same hardness to the sound; it's an odd thing to like i guess, but i dig it and haven't found anything else* that does the same thing.
i'm including big marshall and mesa rackmount power amps (strategy 500 FTW), small valve amps with valve or SS recs that i've near-incinerated, mesa rectos in either mode, the obvious heads and speakers, various power tubes, and of course a host of highly underrated solidstate amps.
enter a 1980 (birth year) bandmaster reverb...

i know it has 4 less 6L6s than the mighty world-eating supertwin, but i also know that i like this circuit.
and it could be a great test bed for some self-education, ie adding a bias pot, tube rec, new transformers, etc.


or

?

[crusty philtrum]

Re: amp gurus: how mod-friendly is the 70W ultralinear bandmaster circuit?

( Disclaimer : Just because i'm responding here doesn't mean i think of myself as a guru of any sort)

I am familiar with these things. I owned a Super Twin in the early 80s .... as you know, they're very loud and very clean (make great bass amps). I've worked on the 70 watt version in the past.

Firstly, is yours set up to run on 240 Volts ? (i.e. have you used it ? If not, does it have a voltage selector somewhere on the chasis ?) If you can run it here, you won't need to change the power transformer.

It is possible to disconnect the ultra-linear wiring of the output stage and run it in the more traditional way, although an extra filter cap would need to be added, and if my memory serves me correctly, there's not a lot of spare space to install another one, some serious improvisation would be needed (the filter cap will have the high DC voltage on it so it must be mounted safely and securely).

Changing the power transformer would probably be necessary if you wanted to add a valve rectifier ... either that or you'd need to add another smaller transformer to provide the 5 Volts needed for the rectifier heaters (the other valves are heated by 6.3 Volts). There is probably not space to fit an additional transformer, so the bigger and more expensive method of changing the power transformer is the most likely option. (it is possible to add a couple of high-wattage resistors to emulate tthe sag of a tube rectifier).

It's also possible to convert the operation of the power valves from pentode to triode to lower the output and soften the sound (earlier breakup).

Other tricks could help to mellow the amp, like reducing the amount of negative feedback (a pot could be installed to give variable negative feedback settings) and trying a 12AX7 in the PI position instead of the usual 12AT7.

I guess it all depends on how much money you want to throw into it and how far you want to go with the changes. I'll try to get you on the 'phone at some stage if you want to discuss the various possibilities.

[dr. ad]

Re: amp gurus: how mod-friendly is the 70W ultralinear bandmaster circuit?

taking all of this on board, i think it would be wise for me to pass on this one.
the idea of further mutating the CBS ultralinears is well out of my reach, at this stage.

thanks for all the info guys, i bookmark this stuff

[Chickenwings]

Re: amp gurus: how mod-friendly is the 70W ultralinear bandmaster circuit?

Ad...they still sound pretty damn good! My neighbor has a bassman 135 and it sounds wonderful, ive played guitar thru it too. Great amps. Just cos it doesn't pass the internet solder sniffer test does not meat that UL fenders can't sound awesome just as they are. remember...there was a time when this design was considered an improvement. You don't need to replace all the iron etc. Its just that there is a current trend of people who are obsessed with the almost mythical "POWERRR TUUUUBE BRRREAKUPPPP" (said in a god like voice with heaps of reverb and tape acho). Biased right with strong tubes and thru your speakers of choice they are brilliant.

[guitfiddle]

Re: amp gurus: how mod-friendly is the 70W ultralinear bandmaster circuit?

Ad...they still sound pretty damn good! My neighbor has a bassman 135 and it sounds wonderful, ive played guitar thru it too. Great amps. Just cos it doesn't pass the internet solder sniffer test does not meat that UL fenders can't sound awesome just as they are. remember...there was a time when this design was considered an improvement. You don't need to replace all the iron etc. Its just that there is a current trend of people who are obsessed with the almost mythical "POWERRR TUUUUBE BRRREAKUPPPP" (said in a god like voice with heaps of reverb and tape acho). Biased right with strong tubes and thru your speakers of choice they are brilliant.

Once again, it goes back to people not regarding the cleans of an amp enough. Overdrive is cool, but a killer clean tone is more useable to some folks. I had to play a gig a couple of weeks ago with a 5 watt tube amp, and to get the volume I needed to be at, it was mush. It sounded much better turned down, and fed through the PA, but on it's own it was pretty useless, cause the excess power tube overdrive at that level sounded terrible.

[dr. ad]

Re: amp gurus: how mod-friendly is the 70W ultralinear bandmaster circuit?

i appreciate the advice.
from experience, i believe that these ones aren't about overdrive; they're not the kind of amps i'd want to run on 10 and control it with the guitar
; ]

the 'cold feet' is more to do with me today picking up a crate V30 quad for 300 bucks hehehe
and that the schematic frightens me. which suggests that getting one of these with the intention of converting it to KT66s and adding a valve rec is a bit ambitious of me at this point.

i'm torn between thinking that i'll:
1) get this beast, run my v-twin through it, and be happy enough to sell off some gear
2) have a big heavy 110V piece of furniture that i don't bond with (because it was actually having 6x6L6s that i liked, and not the very hard-sounding preamp), and can't offload because it's not from the late 60s/early 70s.

so indecisive! i should put a skirt on.