Converting Eico HF-81 to a simple PP EL84 (pics incl.)

[cjkpkg]

Here is my build from the Tubecad Blog circuit

Additions are:
CCS on the EL84 cathodes
Red LED on the input tube cathodes

 

[Eli Duttman]

The OP stated the choke was OK. No choke is shown in the HF-81 schematic. Is the power trafo intact?

The idea of switching to 12DW7s is very good. However, there are plenty of sockets available on the "carcass". So, use system # 1 of a 12AX7 (voltage amp) and a 12BH7 (splitter) in a channel and system 2 of the same 2 types in the 2nd channel. When tubes wear out, flip flop. The 'U7 triode makes an acceptable "concertina" phase splitter, but it is out classed by the more linear 12BH7.

 

[ArtG]

Eli-Would the 12BH7 be a "direct swap", or would value changes be necessary to the plate and cathode resistors, for best performance?

 

[ArtG]

Eli-Would the 12BH7 be a "direct swap", or would value changes be necessary to the plate and cathode resistors, for best performance?

 

[Eli Duttman]

The 12BH7 triode is close enough to the 12AU7 triode to permit direct insertion (assuming the doubled heater draw is not problematic), but tweaking things to maximize performance is a DIYer's prerogative.

 

[ARIYAHOOR]

can original schematic of HF-81 be modified as following??
no phono stage
no FM/AM input
no pre out
no Mic in
no tone control
no input selector (single input)
no function selector


the only remaining sections are line stage + amplification

 

[cyclecamper]

I'd turn it into a guitar amp.

 

[directdriver]

can original schematic of HF-81 be modified as following?? no phono stage, no FM/AM input, no pre out, no Mic in, no tone control, no input selector (single input), no function selector. the only remaining sections are line stage + amplification

Before you ask all these questions and go any further, ask yourself if you know how to read a schematic or at least understand the idea of signal path. Because I just posted a schematic that's exactly as described above! Check post #20. Bypassing unneeded functions in an integrated amp is really easy... you just don't connect the wires to them.

 

[ARIYAHOOR]

Before you ask all these questions and go any further, ask yourself if you know how to read a schematic or at least understand the idea of signal path. Because I just posted a schematic that's exactly as described above! Check post #20. Bypassing unneeded functions in an integrated amp is really easy... you just don't connect the wires to them.

Oh for sure... I can read schematics but I'm not so familiar with tube-based ones... I used to fix black and white T.Vs... one way or the other I could read the scema and follow the signal path... the point is that I didn't know if the circuit is Ok if I disconnect any section or not... I'll try to omit phono stage section at embryo stage

 

[Eli Duttman]

Repeat!! Is the power trafo intact?

 

[ARIYAHOOR]

Repeat!! Is the power trafo intact?

Oh yeah works perfectly (tested)

 

[Triodethom]

I have two of theses what value is the choke you speak of ?

 

[directdriver]

Here's another way to turning it into a pure amp with the least butchering, easily reversible. Just use the "Tape Out" jacks as inputs and add input resistors and, voila, you have a non-integrated amp..... or segregated amp?

 

[ARIYAHOOR]

Forgive me for saying this, but I think that you have unrealistic expectations. Almost everyone on this forum has a life somewhere else that takes most of their time. Many will only look into the forum once a day, and often rather less. Of those who had viewed your thread, I would think that most would not know what an Eico HF-81 amplifier is and so they would not reply (I certainly have never heard of that model). To express disappointment at a lack of response after only 5 hours from your original post seems completely unnecessary.

Oh come on... don't be like that I was just kidding ... accept of humble apology if my sense of humor annoyed you

View attachment 287374

hope this helps

Oh thanks fella

Okay then, just build it like the circuit and use whatever existing parts you can salvage from the Eico. If you plan to use the Eico chassis then you can't use an octal tube like 5AR4 without drilling a bigger hole for its socket. I would just add a filament transformer so you can re-use the dual EZ81/6CA4 rectifier tubes.

Here's the schematic for the HF-81 and you will notice the Pilot amp section uses the same tube complement as the EICO so it's not that hard to implement the circuit. The Pilot is the best sounding 6BQ5 amp I have ever heard, period.

thanks for shcema ... I insist to use ez81 rectifiers

I have two of theses what value is the choke you speak of ?

I don't know... google it and read the articles in sterophile

Here's another way to turning it into a pure amp with the least butchering, easily reversible. Just use the "Tape Out" jacks as inputs and add input resistors and, voila, you have a non-integrated amp..... or segregated amp?

Oh it's a moot remedy... I should follow signal path in the way you said
thanks anyway

 

[Triodethom]

There are no chokes in the hf-81 stock

There are NO choke in the stock hf-81 . Thanks for the pointless redirect to the stereophile article . You may find in this forum if you want some one to read an article the best thing to do is 1.provide a link and 2. have answer the question . How exactly did you determine the choke was ok and you are unable to determine the value of said choke. Regards

 

[ARIYAHOOR]

There are NO choke in the stock hf-81 . Thanks for the pointless redirect to the stereophile article . You may find in this forum if you want some one to read an article the best thing to do is 1.provide a link and 2. have answer the question . How exactly did you determine the choke was ok and you are unable to determine the value of said choke. Regards

Oh I meant power transformer... tested O.K... this is the link in stereophile

 

[directdriver]

The Stereophile article has nothing to do with whether the amp has a choke or not.

The amp does NOT use a choke! So stop mentioning chokes!

 

[tubino]

Here's another way to turning it into a pure amp with the least butchering, easily reversible. Just use the "Tape Out" jacks as inputs and add input resistors and, voila, you have a non-integrated amp..... or segregated amp?

I did just that a long long time ago, and it worked great. Actually, come to think of it, I did it on a Fisher. Maybe an HF-81 too... HF-86 can sound great too, but Eli Duttman is dead on with the 12BH7 idea.