Will this output transformer work?

[TarheelTechinTraining]

I recently acquired a OT from a JTM 60.
I would like to use it to build a jcm800 clone 50watter. I know the jtm60 OT is 6k on the primary side and only has an 8 ohm and 16 ohm on the secondary side. The jcm800 calls for 3.6k on the primary and has 4,8,& 16 ohm taps. My basic question is will it work? Thanks in advance, greg

[Chuck H]

It will work fine. It won't be a JCM800 however (since those run a 3.6k primary). IMHO it'll be great. I've use a 6.6k primary OT's for EL34 amps between 400Vp and 500Vp a few times with very good results. So... A little different, but still the same. The tubes will actually run a little easier and the tone may be a tad more compressed sounding than with the lower primary impedance IME. That can't hurt since 800's can be spikey and harsh at their worst (gritty and brash at their best).

[g1]

How long it will last doing "JCM800 duty" is another question entirely .

[Chuck H]

How long it will last doing "JCM800 duty" is another question entirely .

Well, if you look at the reason most Marshall OT's leave the room "feet first" I don't see any reason an open load on a 3.6k primary OT should be any better than an open load on a 6.6k OT

[TarheelTechinTraining]

I'm sorry I'm not familiar with the term " open load". What is it? Also do I run the NFB to the 8 ohm tap? Thanks so much for everyone's help and direction,Greg

[Chuck H]

An open load is just NO load. Not a monster that hides inside Marshalls Just make sure the amp is plugged into a speaker.

Since the 8ohm tap is what you have, use it. I've used the regular Marshall 100k/4.7k (21/1 ratio) off the 8ohm tap (rather than the 4ohm as per the schematic) and didn't notice as much difference as I expected. If you really want to get closer to the stock feedback voltage you could add a 47k resistor in series with the 100k, but I might just use a single 120k resistor and call it close enough.

[TarheelTechinTraining]

Thanks so much for clarifying that Chuck. I'm a novice on tubes but no load is badddd. This is my first build and is it a Frankenstein. Think I'll call her Francis.

[Chuck H]

I never named my amps. I always kept it on the DL early on when I was doing mostly mods to existing amps. At that time I was a decent player and gigging. My amps (Marshalls at that time) were always modded for gain and to my personal tastes, so they were particularly suited to my style and sound. I think the right tone for the right player is the real secret to good tone. So I always came across as having good tone. I also built my guitar from Warmoth parts with top end electronics and trick wiring. Other players would often rave and ask how I got my tone and I'd just say "I dunno.?. It's just a strat plugged into a Marshall." Later I did start to bring home made looking projects on stage and it was just as fun talking to other players from a builders perspective. The best were jam nights when other players got to play through my amps so I could hear them from the audience. There's a lot to be learned from that and the kudos from players were a nice perk.

[TarheelTechinTraining]

Thank you for the encouraging words Chuck.
While I'm picking your brains. Should I use a 3h choke , 5h choke, or a resistor?

[Chuck H]

Most of the Marshall chokes I know of are pretty burly. I don't imagine any were 3H. I think some replacement 800 chokes are 5h. You need the choke to get the faster response and attack envelope the 800's are known for. The bigger henries will offer better ripple rejection, so, less "beating" under heavy clipping. Also part of the sound. So I'd say 5h choke.

EDIT: "beating" is that low frequency warble that sometimes accompanies heavy clipped tones. Seems especially noticeable with Marshalls. It's an overtone caused by the marriage of any ripple present and whatever note/s are played.

[überfuzz]

Seems we drift off topic here. Ah well...

In general the series resistance of a choke is often quite bit lower then that of a resistor used as a ripple smoother. Hence, a resistor works fine, but in general it lowers the voltage more than a choke do. Do you know the voltages at the secondary taps? Are you high or low (voltage) after the rectifier?

[TarheelTechinTraining]

It seems to be low , about 400v after rectification.

[überfuzz]

You've got any idea of what kind of power stage you're aiming for? You can have much fun with 400 Vdc... One of my best sounding home-brews has a B+ just shy of 400 Vdc, it sags to roughly 375 Vdc when pushed to, once again, roughly 35W output.

[TarheelTechinTraining]

I'm not really sure what I'm aiming for in the power stage. Do you mean wattage wise? I'd say about 20 to 25. Like I said I'm still learning. Thanks for the help ,greg

[überfuzz]

If this is you first build I'd encourage you to try some fender circuit. Vintage fender circuit are very popular among amp tinkerers and you'd be able to get a lot of help, when, if, needed. Do you have any preferences when if comes to guitar sound(s)? Do you have any idea whether you and your band play loud or not (kind of pivotal when if comes to when you want the amp to break up, i.e. what wattage you're want)?

I'd say not to worry about the 400 Vdc you see after the rectifier. It is probably more than or well enough for this project. Right off the bet, if I had a set up with 400 Vdc B+ I'd probably go for a pair of 6L6 tubes in the power stage.