I have a EICO ST-70 and a pair of EICO HF-20, both are tube amps.
I wonder is it a good idea to mod either ST-70 or HF-20 into FISHER KX-200's schematics?
BTW, any idea whether the OPT of EICO ST-70 or HF-20 handles 50 Watt output? I guess HF-20 OPT might be fine since EICO HF-50/52 uses the same OPT for 50 Watt power. I am not sure ST-70's OPT can handle 50 Watt.
I wonder is it a good idea to mod either ST-70 or HF-20 into FISHER KX-200's schematics?
BTW, any idea whether the OPT of EICO ST-70 or HF-20 handles 50 Watt output? I guess HF-20 OPT might be fine since EICO HF-50/52 uses the same OPT for 50 Watt power. I am not sure ST-70's OPT can handle 50 Watt.
Hi genome,
Why on earth would you do that to a very good sounding piece of equipment?
I have an Eico ST-70 that I will be rebuilding as stock. I've rebuilt several Eico products and they have always sounded excellent when all was said and done. Eico does tend to use excellent iron in their output transformers.
I have some Fisher as well. Excellent product, but for an amplifier you would pretty much need the Fisher output transformers in order to make it sound the same.
My own personal feelings are to simply restore what you have. Additionally, they retain value much better than anything modified to be something else. I think you will end up with better sounding equipment by building what you have.
You're lucky to have the two HF-20 amplifiers. Don't spoil them by trying to make them into something else.
-Chris
Why on earth would you do that to a very good sounding piece of equipment?
I have an Eico ST-70 that I will be rebuilding as stock. I've rebuilt several Eico products and they have always sounded excellent when all was said and done. Eico does tend to use excellent iron in their output transformers.
I have some Fisher as well. Excellent product, but for an amplifier you would pretty much need the Fisher output transformers in order to make it sound the same.
My own personal feelings are to simply restore what you have. Additionally, they retain value much better than anything modified to be something else. I think you will end up with better sounding equipment by building what you have.
You're lucky to have the two HF-20 amplifiers. Don't spoil them by trying to make them into something else.
-Chris
The OPT in the HF-20 (which is the same OPT used in the HF-22 and HF-35) most definitely cannot handle 50 watts. In fact, it barely squeaks by with EL-34 outputs in the HF-35. The HF-50 uses a similar looking potted Chicago OPT but it is a different beast inside; it can handle 50w but good luck finding a pair. The HF-60 uses the Acrosound TO-330 which can handle 60w.
I wouldn't do anything to an ST70 other than retube and replace the filter caps. Wish I still had mine.
Thank you for all your advice. I will not mod the circuits.
But due to the age, my EICO ST-70 and both HF-20 need a big rebuild. They certainly need new filter caps. Some coupling caps need to be replaced too since the bias is off. At least some resistors may need to be replaced; I noticed a few resistor values are off. I don't know how to start this task.
When you replace the filters/caps, do you just add the new filters in parallel to the old ones, or remove old filters first and then solder back the new ones? I guess by no means I have to remove the old resistors before installing new ones.
Any suggestions?
But due to the age, my EICO ST-70 and both HF-20 need a big rebuild. They certainly need new filter caps. Some coupling caps need to be replaced too since the bias is off. At least some resistors may need to be replaced; I noticed a few resistor values are off. I don't know how to start this task.
When you replace the filters/caps, do you just add the new filters in parallel to the old ones, or remove old filters first and then solder back the new ones? I guess by no means I have to remove the old resistors before installing new ones.
Any suggestions?
Also, don't forgot that the PT of the HF-20 is the weak point to squeeze more potential from the OT. The HF-35 uses the same PT as HF-50, HF-52, HF-60. IMHO, the pre section of the HF-20 is only average, I will rewire it to a HF-22 and uses a separate pre.
Take a look at the ST70 mods here. One mod is very drastic and involved, and the other is more like correcting/fine tuning the circuit. I did the simpler change and can say it was well worth a weekend's work.
I have a second ST70 partially recapped and collecting dust. I am tempted to hotrod it just to see what the hotrod mod is all about.
Notice that the feedback compensation is typically tuned according to the OT. Dropping a different OT to replace
the original may result a loosely working amp at best and stability issue at the worst. IMHO, the fisher is no better than the ST70 even before the above mods. So, forget about the Fisher conversion, it will be a waste work. If 50 watts is what you go after, just hotrod the ST70. Do keep in mind, 35W and 50W is like 2db different only .....
Take a look at the ST70 mods here. One mod is very drastic and involved, and the other is more like correcting/fine tuning the circuit. I did the simpler change and can say it was well worth a weekend's work.
I have a second ST70 partially recapped and collecting dust. I am tempted to hotrod it just to see what the hotrod mod is all about.
Notice that the feedback compensation is typically tuned according to the OT. Dropping a different OT to replace
the original may result a loosely working amp at best and stability issue at the worst. IMHO, the fisher is no better than the ST70 even before the above mods. So, forget about the Fisher conversion, it will be a waste work. If 50 watts is what you go after, just hotrod the ST70. Do keep in mind, 35W and 50W is like 2db different only .....
Hi genome,
The rebuild is well worthwhile. For starters, take a bunch of digital pictures of the chassis. All new parts need to be in the same physical locations.
Two ways to do this.
1.) Strip the unit of the old parts and rebuild fresh as if it was a kit (some were assembled by their owners. The owners manual is a kit build manual as well. Some times it's difficult to remove the components from the terminals they are soldered to. A big solder sucker is your friend here, ans some solder wick. Cut the component leads and work the attached part loose, then cut it in the loop to free it.
2.) Replace everything. Carefully remove all wires for the transformers. Mark which colour goes where as you will have two the same colour on the output transformers. Detach components that are soldered to the chassis or tag strips on the chassis. Prewire the tube sockets, then remove the old ones and place the new ones in place. Some of the wiring is now done when you could easily get to the socket, or you can mount the new sockets and wire them from there. Heater wires first - just follow the manual.
Components. Do not attempt to use parts that are in a different package than the old ones. Use axial capacitors for coupling and do not change the values up. 0.0047 uF is the same as 0.005 uF. You can get polypropylene capacitors in axial cases. But either metal film resistors, or metal oxide for power supply components. You will have to use 1 watt or higher in order to get the higher voltage breakdown ratings you need, and to make the package large enough so the leads reach where the old ones were. Note that some 1/4 watt resistors have voltage breakdown ratings as low as 50 volts!
This is a bit of work, but you should be proud of the fact that you just built an entire amplifier that sounds great. I do all my rebuilds this way in case you're wondering. My first was an Eico HF-87 about 20 some odd years ago, and we still use it. It is one of our best sounding amplifiers. (35 wpc) It drives my PSB Stratus Gold speakers just fine, so you don't need tons of power in order for it to use modern speakers. Although, if I had an 80 wpc amplifier, it would be nice to get silly with every now and then.
The only thing I can add to that is if you do good work, your amplifier will last you many years and will probably be in your estate when you aren't around to enjoy it any longer.
-Chris
The rebuild is well worthwhile. For starters, take a bunch of digital pictures of the chassis. All new parts need to be in the same physical locations.
Two ways to do this.
1.) Strip the unit of the old parts and rebuild fresh as if it was a kit (some were assembled by their owners. The owners manual is a kit build manual as well. Some times it's difficult to remove the components from the terminals they are soldered to. A big solder sucker is your friend here, ans some solder wick. Cut the component leads and work the attached part loose, then cut it in the loop to free it.
2.) Replace everything. Carefully remove all wires for the transformers. Mark which colour goes where as you will have two the same colour on the output transformers. Detach components that are soldered to the chassis or tag strips on the chassis. Prewire the tube sockets, then remove the old ones and place the new ones in place. Some of the wiring is now done when you could easily get to the socket, or you can mount the new sockets and wire them from there. Heater wires first - just follow the manual.
Components. Do not attempt to use parts that are in a different package than the old ones. Use axial capacitors for coupling and do not change the values up. 0.0047 uF is the same as 0.005 uF. You can get polypropylene capacitors in axial cases. But either metal film resistors, or metal oxide for power supply components. You will have to use 1 watt or higher in order to get the higher voltage breakdown ratings you need, and to make the package large enough so the leads reach where the old ones were. Note that some 1/4 watt resistors have voltage breakdown ratings as low as 50 volts!
This is a bit of work, but you should be proud of the fact that you just built an entire amplifier that sounds great. I do all my rebuilds this way in case you're wondering. My first was an Eico HF-87 about 20 some odd years ago, and we still use it. It is one of our best sounding amplifiers. (35 wpc) It drives my PSB Stratus Gold speakers just fine, so you don't need tons of power in order for it to use modern speakers. Although, if I had an 80 wpc amplifier, it would be nice to get silly with every now and then.
The only thing I can add to that is if you do good work, your amplifier will last you many years and will probably be in your estate when you aren't around to enjoy it any longer.
-Chris
Even this partial schematic shows an (IMO) obvious area of improvement. Work up a DC heater supply for the low level mag. preamp. If AC heating is retained, use the Sovtek 12AX7LPS in those sockets. The 'LPS is a genuine 7025 equivalent and contains a spiral wound, hum bucking, heater.
In units from this time frame, I have a strong preference for source selector modification. Tape head I/Ps are passe, in today's environment. "Hard" wire 1 set of RCA females to the low level mag. preamp, which is permanently configured for RIAA. Switches in mV. level signal lines is asking for trouble. Make all other I/Ps line level.
Add the series SS diode tweak to the 5AR4 socket. If you have an OS Mullard, it may last a bit longer and you can install a Sovtek replacement in complete confidence.
In units from this time frame, I have a strong preference for source selector modification. Tape head I/Ps are passe, in today's environment. "Hard" wire 1 set of RCA females to the low level mag. preamp, which is permanently configured for RIAA. Switches in mV. level signal lines is asking for trouble. Make all other I/Ps line level.
Add the series SS diode tweak to the 5AR4 socket. If you have an OS Mullard, it may last a bit longer and you can install a Sovtek replacement in complete confidence.
Eli's suggestions are excellent, I will add a few:
As Chris said, take the time to "reassemble", don't rush. You will be surprised how much good practice brings to the table.
- Don't use any cap larger than 50uF after the 5AR4. I use 40uF and am still using an Eico 5AR4 (Mullard made??) that is tested as new.
- The electric outlet that we have these days are usually 120VAC or higher, so there is enough excessive volts for another level of filtering. For example, add a choke after the 1st cap is an easy improvement.
- Redo the rounding to eliminate the chassis/socket grounding everywhere and pay attention to the wiring.
As Chris said, take the time to "reassemble", don't rush. You will be surprised how much good practice brings to the table.
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