Converting Eico HF-81 to a simple PP EL84 (pics incl.) - Page 3 - diyAudio
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Old 15th June 2012, 12:07 AM   #21
cjkpkg is offline cjkpkg  United States
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Here is my build from the Tubecad Blog circuit

Additions are:
CCS on the EL84 cathodes
Red LED on the input tube cathodes
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Old 15th June 2012, 01:15 AM   #22
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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.
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Old 15th June 2012, 02:19 AM   #23
ArtG is offline ArtG  United States
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Eli-Would the 12BH7 be a "direct swap", or would value changes be necessary to the plate and cathode resistors, for best performance?
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Old 15th June 2012, 02:34 AM   #24
ArtG is offline ArtG  United States
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Eli-Would the 12BH7 be a "direct swap", or would value changes be necessary to the plate and cathode resistors, for best performance?
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Old 15th June 2012, 02:41 AM   #25
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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.
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Old 15th June 2012, 07:04 AM   #26
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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
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Old 15th June 2012, 08:28 AM   #27
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I'd turn it into a guitar amp.
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Old 15th June 2012, 03:06 PM   #28
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Quote:
Originally Posted by ARIYAHOOR View Post
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.
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Old 15th June 2012, 06:04 PM   #29
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Quote:
Originally Posted by directdriver View Post
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
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Old 16th June 2012, 01:01 AM   #30
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Repeat!! Is the power trafo intact?
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