Eico HF-20 to bass guitar amp

[hearforever]

A friend gave me an Eico HF-20 recently and it is in pretty decent shape– it needs to be recapped and the phono stage has some distortion.

Since I don't have any need for a hi-fi amp I was thinking I might try to rewire this for bass guitar, something along the lines of an Ampeg B-15. I thought I would replace the selector switch with a 1/4" input and remove some gain stages and also add a James tone stack with the B-15 values. Does this seem do-able?

Should I leave the power-amp section as is? I thought I could leave everything after the first 1/2 of V3 as is and rewire V1 and V2 as separate channels as in the B-15.

I have rewired many PA amps but this would be my first attempt at repurposing a hi-fi amp--- any suggestions?

Thanks

schematic: http://www.acousticpsychos.com/Files/Eico/EICO_HF20S_SCHEMATIC.GIF

 

[kevinkr]

Changed my mind and deleted my previous post after looking at the B15 design. Perhaps with the appropriate speaker this HF 20 might do OK as a bass amp. The B15 claims 45W which might or might not be optimistic compared to the HF20s rated power. You could probably reconfigure the phono stage as an input amp for your bass if you need some additional gain, (you might not) and tweak the values in the baxandall tone control circuit to be suitable for use with your bass. (The baxandall may actually be better in this application than the James, and if you don't like it you can change over later.) Some improvements in the power supply might be worthwhile as well.

 

[hearforever]

I think I will recap and try to get rid of the noise on the phono input and then see how it is and adjust tone controls as needed.

What improvements to the power supply would you suggest? Higher value capacitors?

Thank you

 

[kevinkr]

I think I will recap and try to get rid of the noise on the phono input and then see how it is and adjust tone controls as needed.

What improvements to the power supply would you suggest? Higher value capacitors?

Thank you

You will need to remove all of the EQ in the phono stage first in order to use it as the input amplifier for your bass.

BTW what kind of bass do you have, upright or 4 - 6 string bass guitar, and active or passive pickups?

I'd probably go with a 5AR4 rectifier, (add a UF4007 in series with each plate lead) and make the first supply cap (C26A) at least 30uF, but no more than 40uF.. I might change R31 to a choke with a dcr a couple of K ohms, and an inductance of maybe 10H or so - this would stiffen the screen supply a bit and reduce the ripple on the screens helping to reduce potential hum levels - if you do this you may need to increase the cathode resistance in order to keep the quiescent point within reasonable limits, and it may effect loop gain slightly. I'd probably change C14 to something like 220uF.. Note that other than the capacitor change I would try each one of these suggestions one at a time and see if you like the result before moving on to the next change. While technically OK maybe some of my suggestions might not result in the sound you are looking for or worse. 😱 😀 If possible use the speaker you plan to use long term with this amplifier..

 

[hearforever]

I will be using my mid 60's fender mustang bass with this amp. It will just be a practice amp/recording amplifier. I will probably just be driving a 12" speaker and also hoping to use the tape output to record direct into my audio interface.

by EQ in the input you are talking about C22 on the schematic? The 68k R39 grid stopper seems appropriate and maybe I would change R40 to something like 1M?

Still haven't test it out yet– I thought I would rewire the input see how it sounds stock and then move on to the tone stage.
Is the 5AR4 a drop in replacement for the 5U4?