Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: six el six on September 30, 2010, 02:34:57 pm

Title: jcm 900 model 4100 output transformer
Post by: six el six on September 30, 2010, 02:34:57 pm

I've tested the output transformer on this amp using the geofex testing procedure and one primary halves resistance  is 50% less than the other. Placing 3v ac on the secondary 8 ohm tap gave two different readings on each half of the primaries. Again one halves reading is half of the other one. I couldn't find anything wrong with the secondaries at all. I've done this same test on many output transformers and I feel pretty comfortable concluding it's bad. I suspect that the bad speaker cab connection I discovered on this person's rig caused the load to vary intermittently resulting in the malfunction. Question #1. Does this make sense? Question #2. Does anyone have any suggestions on specific output transformers I can recommend to this person other than Mercury Magnetics "direct drop in replacement". Can I use something like a JCM 800 100 watt replacement from one of the less expensive manufacturers? Thank you for your help.

Title: Re: jcm 900 model 4100 output transformer
Post by: FYL on September 30, 2010, 02:52:53 pm

What you describe is alas quite common : Marshall used less than optimum OTs.

Doug's 100-watt OT can be used with minimal chassis mods.

http://www.hoffmanamps.com/MyStore/perlshop.cgi?ACTION=enter&template&thispage=Transformers

Title: Re: jcm 900 model 4100 output transformer
Post by: six el six on September 30, 2010, 07:59:40 pm

Okie dokie! Thanks for the tip.

Title: Re: jcm 900 model 4100 output transformer
Post by: shortfuse on September 30, 2010, 11:03:40 pm

Let us know how it sounds if you use one of Doug's I have been debating on getting one of his for my 900 because he Mercury is like twice as much and I am not sure I will hear twice as much a difference.  But I have a 50w 4500 with 5881's

Title: Re: jcm 900 model 4100 output transformer
Post by: dynaman1 on October 01, 2010, 06:01:52 am

Yeah, that's weird. I picked up a used pair of late model Marshall 100W transformers and had a miserable time identifying the leads (tabs). I recall having the same problem, but wound up figuring it out. Check this Hammond diagram: http://www.hammondmfg.com/pdf/EDB1750X.pdf

I'm willing to bet your transformer is the same as mine. Check this picture of the amp I made: http://www.myspace.com/andryamp/photos/25102436

It's wired for 4, 8, 16 operation. On the "primary" side of the OT, You'll notice two unused tabs. IIRC, the one that's fed with a heavy gauge copper wire is the 2 ohm tap and I think the other unused tab is an oddball tap on the primary. I do remember isolating the primary to FOUR (not three) tabs. One of those tabs in NOT the center or either end of the winding. I'm willing to bet that you've simply been taking measurements from the oddball tab.

Shoot me your email if you want some bigger pics of my transformer and wiring. I'd hate to see you toss out a perfectly functioning OT. 

Title: Re: jcm 900 model 4100 output transformer
Post by: dynaman1 on October 01, 2010, 06:16:57 am

I'll apologize to Doug in advance for posting a link to Hammond. I only did that for technical reference.

I'll also pay penance by ordering Hoffman turrets today.  

Title: Re: jcm 900 model 4100 output transformer
Post by: six el six on October 02, 2010, 07:37:42 pm

The person I'm doing the repair for wants the mojo 900 tranny because it fits. Any thoughts/experience concerning that product?

Title: Re: jcm 900 model 4100 output transformer
Post by: FYL on October 02, 2010, 08:17:05 pm

Quote

The person I'm doing the repair for wants the mojo 900 tranny because it fits. Any thoughts/experience concerning that product?

No direct experience. Made by Heyboer so it's probably excellent.