Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dwp on May 12, 2010, 11:57:00 pm

Title: converting 4-EL84 to KT66.
Post by: dwp on May 12, 2010, 11:57:00 pm

Hello all, I have this 4 EL84 build I just can't make peace with. Is Mercury Magnetics Vox repro iron. The PT (V-30H-M) is not a fat stack, but it should handle pair of KT66's, yes?
Though they don't publish their data on their wiring sheets, 4 EL 84 OT (VXO30M) should be 4K input, close enough for KT66? (5K?)
      Also have 4-8-16 output, so I get confused on which way to double. If I want to run 6V6, 8K, does my 4-8-16 become 2-4-8, or did I get it backwards?
Thanks in advance, still at the connect the dots level of electronic engineering.
My build is basic 5F6A/Plexi, MV, with EF 86 channel seperate.

Title: Re: converting 4-EL84 to KT66.
Post by: sluckey on May 13, 2010, 06:58:29 am

Quote

Also have 4-8-16 output, so I get confused on which way to double. If I want to run 6V6, 8K, does my 4-8-16 become 2-4-8, or did I get it backwards?

The OT impedance ratio is constant, so if you double the primary side Z, the secondary side Z will need to double also.

Title: Re: converting 4-EL84 to KT66.
Post by: dwp on May 13, 2010, 11:43:35 pm

ouch, you're going to make me think.......(lol) Thanks for the reply. So any opinion on converting the 4 EL84 "VOX" style to KT66? Think it will darken my overdrive somewhat and make the master volume more "useable"?
Steve, am pretty sure I plagerized your November on this build too.
thanks again, dwp

Title: Re: converting 4-EL84 to KT66.
Post by: darkbluemurder on May 14, 2010, 02:26:47 am

As regards heater current you should be OK - 4 EL84 = 4 x 750mA = 3A, 2 KT66 = 2 x 1.3A = 2.6A. Of course you need to move to octal sockets.

As regards OT impedance matching: I have seen OTs recommended for 2 KT66 specified at 6.6k and 8k primary. If yours has 4k you need to move down one notch at the impedance selector, e.g. have an 8 ohms speaker, set it to 4 ohms.

Alternatively you could convert to 2 EL34 and keep the transformer as is. I believe Matchless uses the same transformers for their 4 EL84 and their 2 EL34 models.

Cheers Stephan

Title: Re: converting 4-EL84 to KT66.
Post by: Shrapnel on May 14, 2010, 03:57:18 am

Don't forget to adjust the PI to a more standard one, as the LTPI for EL84s are set up for less drive. :icon_jokercolor:

Title: Re: converting 4-EL84 to KT66.
Post by: dwp on May 14, 2010, 08:30:59 am

Thanks for the responses. I got 5K off a data sheet somewhere, But I think you may be right, Radiospares for original JTM45 was either 6.6 or 8K.....noted on PI.

Thanks again, dwp