Eico HF86 modifications

[83stang]

I want to build an Eico HF86 but want to change the tubes to EL34's to get more output power, how would i accomplish this? and what power supply would supply a stereo version of this amp? What Hammond power/output transformers would work for this?

Thank you for the help, the link for the schematic is here: http://www.triodeel.com/eicohf86.gif

[Eli Duttman]

That amp uses small signal circuitry along the same lines as Dyna, Fisher, and Scott used. It's a voltage gain block DC coupled to a "concertina" phase splitter. The 7247/12DW7 is 1/2 a 'X7 and 1/2 a 'U7. It's fine for driving EL84/6BQ5 "finals", but it's not up to driving EL34s. If you want to retain that specific small signal topology and obtain more O/P power, follow the lead of Fisher and Scott, by using 7591s as the O/P tube type.

OTOH, if you are locked in to EL34s for the "finals", that topology style is what the Dyna ST70 uses. A pentode voltage gain block yields the requisite extra gain. Compare the Dyna ST35 to the HF86. You will see similarities. Now, look at the ST70.

[Robert McLean]

If it is something about EICO that you like, then the HF22, HF35, HF50, HF60, HF87, and there is an EICO ST70 as well apparently, they all use EL34.
I dont know anymore about them than that, you can find links to schemtics here

http://www.triodeel.com/schindex.htm

EDIT : DOH ! I just noticed your link, so obviously you know about these already.

[83stang]

I initially wanted to build an ST70 but the transformers are too expensive, so unless you can suggest some Hammond power and output transformers because i have a source of them close to home, then i would make one. any ideas? Thanks

[Eli Duttman]

Hammond's 1650N should prove satisfactory as your O/P trafo.

I don't see anything satisfactory for a stereoblock power trafo from Hammond. IMO, you should bite the bullet and buy AnTek's model AN-4T360, which costs 50 USD.

Unfortunately, 7199s are fast disappearing. To do the Dyna topology, 2X EF86s and an ECC99 are in order. IMO, you will get better sound from a Mullard style circuit that uses a 12BH7 and 2X 12AT7s as the small signal tube complement.

Let's see, each of the AN-4T360's 6.3 VAC windings will easily energize 2X EL34s, a 6CA4/EZ81 rectifier, and a 12AT7. Power is needed for the 12BH7 and the bias supply. One of the "12" VAC windings on an AnTek model AN-0112 takes care of the 12BH7. Derive the bias supply by voltage multiplying the 2nd "12" VAC winding. Use Schottky diodes in the multiplier and be switching noise free.

[83stang]

wow 50 bucks, well after shipping and exchange to here it should be cheaper and more useful then a Hammond power transformer, thanks for the suggestion for the mullard circuit, it's between the mullard and dynaco circuit now for what i want to build. Would the 1650G work in these circuits instead of the 1650N, because I'm a little strapped for cash cause i'm a college student haha.

[Eli Duttman]

Dude,

Guitar amp O/P trafos can't "cut the mustard" in the deep bass dept. and their top end is also suspect. TANSTAAFL!

Perhaps Edcor can help you hold costs down. Their model CXPP60-8-4.2K appears to be suitable.

If you are the proverbial dirt poor student, consider lowering your ambition and building around 6V6s or EL84s.

BTW, what speakers are you intending to use? Have you applied Paul Joppa's 102 dB. Rule?