Monterey amp: #7026, #7027, # ... ?

VintageCharlie · Post by **VintageCharlie** » Wed Jul 20, 2011 10:19 am

Anything regarding the early Hendrix tones and the 45/100 and details on its construction might find a home here.

As for the 16 vs 8 ohm taps, i think in Franks case (as Roe already pointed out) he was experimenting with the negative feedback (if that is the thingie that goes to presence?). 16 should be stock, but Rodger Mayer supposedly modded it to 8 later on. Seems like 8 ohms give a fatter, rounde, warmer, bluesier tone, while 16 ohm can fork out convincing Fillmore East (BOG) tones - this gives a big difference in tone. 8 ohm could bring you closer to that Monterey style full, fuzzy mayhem, when boosted with the right fuzz.

As for the OT's taps - that was just a guess. Roe mentioned it once as a possible reason for that unique tone of #7027 or whatever was the Monterey amp.

Xplorer · Post by **Xplorer** » Wed Jul 20, 2011 10:36 am

something i don't understand is : what's the impedance of the cab, loaded with four 16 ohms speakers , and then why roger mayer would swap the impedance taps from the transformer, on the selector ? isn't it more easy to simply switch with the 2 pins plug ?

then, when i'll understand it better, i'll have a question.

Also, Brian wallace wasn't agree with all that.

from what he knows, the guy claims that the transformer was overwound, or at least, looked overwound. but this guy perhaps compared a 540 v and a 560 v transformer, ( this is my supposition, as franck had a 540 v transformer , that cloned this model of transformer, and it was previously made to replace the original 7027 transformer i think ) and 560 v is normal for this 1202 - 84. so : perhaps "overwound" to have 20 more volts, but normal. This is just a supposition.

what is : playing with the negative feedback ?

maybe a 220pf , 33k ( or 56k , we can't be sure )

and 560pf or no 560pf as seen on the pictures at amparchives. no bright cap on the 7027, while there's one on Franck's Clone.

470k mixer resistors. one 8uf and one 10 uf i think. but i don't know what's the effect of this.

a 4 henrys choke, though Franck has a 10 henrys choke. it's strange.

parkhead · Post by **parkhead** » Wed Jul 20, 2011 11:31 pm

re Negative feedback

the tap the feed is taken from affects the negative feedback

for example on the bassman the amp is 2 ohms so the feedback Ratio is created by the voltage on the tap
and the voltage split between the 27k resistor and the presence setting

on a marshall you can run the amp with a 16ohm speaker but move the purple negative feedback wire to any of the taps on the selector so while your amp may be running a 16 ohm cab on the 16 ohm setting
hooking the purple feedback wire to the 8 ohm tap would feed the network less voltage than the stock 16 ohm tap

with less nagtive feedback the amp is rawer and has more "nasty harmonics"

you can also adjust this by changing the 27k feedback resistor to 47k or 100k as marshall did in the 70's

fwiw I run some of my amps with a 15k negative feedback resistor which is not historically correct but cuts the volume and makes the amp sound fatter under overdrive

people have experimented with this forever & then settled on stock values again

p

VintageCharlie · Post by **VintageCharlie** » Thu Jul 21, 2011 4:06 am

Thanks a lot for explaining this a bit in the detail, parkhead!
So the amp would get "nastier" (odd order?) harmonics with the negative feedback connected to the 8ohm tap or to the 16ohm tap? From Franks clips i would say that 8ohm was A LOT rounder, fuller, warmer if you want - like an earlier Marshall tone and the 16 ohm negative feedback setting sounded like later Marshalls - slimmer, more defined, tougher. (probably also nastier when pushed?)

Roe · Post by **Roe** » Thu Jul 21, 2011 4:56 am

Negative FeedBack (NFB):
27k@16 = 19k@8 = 13.5k@4
27@8 = 38k@16 = 19k@4

If marshall swapped the 8 and 16ohms secondaries by mistake, this means that NFB was 27k@8, significantly less NFT than stock - giving a rawer, louder tone. Also, running a 8 ohms load (two 16ohms cabs) from the 16 ohms secondary will give more power and more 3rds, 5ths and 7ths - sounding dirtier and more aggressive than a stock super 100 amp (or closer to the earliest el34 amps with the 119 ot). This has major impact on tone and feel.

reducing the b+ from 560v to 540v will give a tad more mids, compression and breakup. however, power supply sag is just as important. the big hammertone RS PT was just rated for 200mA (a twin OT is 450mA by comparison, as is George's 1203-80 as seen on wiki), but I don't know what the 1204-43 is rated for. Anyway plexis tend to sag almost 100v under full load, something that contributes greatly to distortion and compression.

Then we have the inductor/choke. Most used a 352-114 rated for 3H, but some used the 20H RS or possibly even an unidentified 10H. The kt66s will draw much less screen current than el34s, being easier on the choke, meaning that the henries will drop less under full load. Personally I prefer the 3H in my super 100 amp, since it sounds dirtier, warmer and rawer - just more like a marshall. But some people prefer the crisper tone of a choke with higher henry rating.

Finally, we have the tonestack. I thing 56k/220pf is a little dark, preferring 270pf (as shown in early schematics). But some people like 33k/500pf

parkhead · Post by **parkhead** » Thu Jul 21, 2011 8:17 am

forgetting about miswire secondaries for a moment I believe that a certain number of classic recorded tones
were achieved via ignorance

ie with the amp selector wired at 16 ohms & two cabinets piled up and plugged in
the way a teenager without access to the internet would do it

how many of these guys ran the things without adjusting the selector ?? we may never know

p

shakti · Post by **shakti** » Thu Jul 21, 2011 8:33 am

That very same thought has crossed my mind many times. Same thing with negative feedback when connected to the speaker jacks - with one cab you've got 16 ohms, with two cabs you've got 8.

parkhead · Post by **parkhead** » Fri Jul 22, 2011 11:15 am

anyone ever played one of these with the Feedback wire connected to the 100ohm tap
or with the 100ohm tap accidentally connected to the 16 ohm selector???

it would be far easier for ROSEMARY to wire the selector wrong than for Drake, Dagnall or RS to wind the transformer completely wrong

likewise it would Rosemary or "Emma" over at the transformer shop swapping the wire colour codes to terminate the finished transformer ... depending on the final test stage
they might simply be hooking up the primaries and looking for a signal on the 8 ohm secondary
if the 100ohm and 16 ohm color codes were swapped the unit might sail right through QC

Just thinking aloud & thinking about what would or would not pass QC if it were a freak transformer

the other option is a special sample from the MFG that was purposely given to the artist as a test
however that would not account for strange behaviour and extra distortion since one would suspect that
they would be looking for improved headroom with any upgraded sample bits

p

parkhead · Post by **parkhead** » Fri Jul 22, 2011 11:26 am

if the negative feedback were increased
the amp would sound fatter but would have obviously lower volume

especilally if the mismatch were to the 100 ohm tap

anyone want to try a 13k or 4.32k feedback resistor in their JTM ?

roe is my math right ?

p

Xplorer · Post by **Xplorer** » Fri Jul 22, 2011 11:27 am

It's not very kind for Rosemary, i'm sure that she was very professional ; )
But she allowed an interesting sound if it's true.
i saw that Franck added a switch for the feedback resistor value, but i don't know much.

parkhead · Post by **parkhead** » Fri Jul 22, 2011 11:32 am

thank you !!

the switch is selecting between two transformer taps right on the selector
& we have 4 choices

100ohm
16ohm
8ohm
floating on the output ie speaker side

p

Roe · Post by **Roe** » Sun Jul 24, 2011 2:06 pm

another pic indicated that it was btw 8 and 16 ohms, not 100V

parkhead · Post by **parkhead** » Sun Jul 24, 2011 4:40 pm

I think we were trying to determine if the special unique sounding hendrix amp with a freak transformer

was an amp where the 16ohm and 100v taps were mixed up

http://vintageamps.com/plexiboard/viewt ... =1&t=66627" onclick="window.open(this.href);return false;

the primary impedance would be 6.25 x too low = severe distortion

the negative feedback would be 6.25 x too high = the amp attempting to reduce distortion

p

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Monterey amp: #7026, #7027, # ... ?

Monterey amp: #7026, #7027, # ... ?

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