First Year JTM 45(1989) Re-Issue Load Ohmage Question/Hotplate

[roeger]

Hi Everyone:....its been a while since i've been here,but nothing like coming to the source for important questions....I've got a gig to go tonight and my first year(1989) JTM 45 RI. has the duty.I have been running the 16ohm tap into an 8 ohm load forever,based on something i learned here a few years back..." the ouput transformers on the early re-issues were wound somewhat incorrectly,the 16 ohm tap into an 8 ohm load is ok ,and actually is a better match load wise and soundwise"....it does sound mighty fine on the 16 ohm tap...i'm using an 8 ohm Marshall 2-12....my question is,i'm also bringing a hot plate,do i use a 16 ohm hotplate,or an 8 ohm hotplate?
thank guys \john r.

[Chuck H]

Obviously your already at the gig. But IMHO if you've been using an 8ohm load and you want the same effect then you want to use an 8 ohm Hotplate. I'm a little skeptical about "the ouput transformers on the early re-issues were wound somewhat incorrectly,the 16 ohm tap into an 8 ohm load is ok". I'd need to see some documentation to get behind this. Some originals used KT66 tubes and others used EL34's. If the same OT was used for both (and I'll bet it was) then what's the confusion. 6.6K primary (which is that amps primary Z IIRC) at that amps Vp for a pair of EL34's, KT66's or even 6L6's is a perfectly acceptible load. Altering the secondary to half the impednce would reflect a 3.3k load on the power tubes. I have noted on one of my own builds that using the 6.6k OT at 3.3k sounded very good. There are considerations about the OT being designed for a specific frequency response at a certain load. That also changes when you alter the primary Z. But if it sounds good and isn't blowing up tubes I think it's fine. And, use the Hotplate of the same ohmage as the speaker load your using.

[roeger]

thanks chuck:i had to make a call,so i reasoned the amp on the 16ohm tap would want to see a 16 ohm hotplate( into the 8 ohm cab).My concern was the amount of time the amp would be running(several hours at 2/3 to 3/4 volume).At home i run the mismatch and i've never had a problem,but not for hours on end.It DOES sound good,its one of the recommendations i remember getting here several years ago,which concurs with what you said......that being said,everything went off without a hitch.I ran the hotplate minimally,-4 ohms/and often 0db reduction(loud drummers!).
The amp is running Valve Arts KT66's.It is a first year(1989), Marshall re-issue JTM45....with the skills i learned here several years ago,i replaced all the caps on the board with sozo's,biased up the valve arts(thankyou Randall Aiken)with a 60's Mullard gz34,a trio of 60's 12ax7's:Amperex in V1,Brimar in V2,Dutch Phillips driver.
The gig was a beautiful farmhouse,in a greenbelt,surrounded by cornfields and untamed fields.Warm sunny,Perfect night for a party.
The tone!magnificent(i played an '87 Lespaul Standard,loaded with handwound paf replicas.)Gigs like this are always awakeup call for me as to why i love vintage circuit tube amps.All analog effects board.Simply glorius snarl,bell-like cleans,growl,slide,rock,blues, folk...played it all...when we finished an electric set,we'd do an acoustic set(no breaks!)...enuf rambling...thanks again...john

[Chuck H]

Sounds like a fun gig. Those types of party/jams were always my favorite things when I was an active player. I actually played one in Santa Cruz California where Neal Young popped in to jam! BBQ, loud tube amps, good players, beer and then Neal Young. Great times.

With the 16 ohm hotplate you get closer to the proper 16ohm load that the amp was designed for the more you increase attenuation. The less you attenuate the more the amp plays into the 8 ohm load of the cabinet. So that seems like a good compomise.

FWIW I think that the confusion on the early JTM45 amps was for those that wanted to use EL34's in them. The KT66 is much happier at a higher primary Z. I think that if you were using EL34's the half impedance thing would be fine. But for KT66's you should really stick to the designed load impedance. But, as I said, if it sounds good and isn't blowing up, it's all good.

[roeger]

good info...thankyou...i was always under the impression that you could not run EL34's in this amp because there was an issue with the amount of screen/grid/filament supply?,with the existing transformer...your santa cruz story is very cool....i did trot out a number of neil young tunes i had been working on last night,to the surprise of my bandmates..while being a die hard electric player for going on 30 years,last year i acquired 2 Martin acoustics...HD28 and HD28V....it has solidified both my electric and acoustic playing...there is more amp maintenance in the near future i'm sure of that,i like doing it myself...thankyou for the feedback..