ab0ez
AB0EZ
That could very well be. I put in a 5 watt 18K resistor in each channel. I'll make some measurements when I get it running.
Eico HF87 rebuild/update
- Thread starter ab0ez
- Start date
That could very well be. I put in a 5 watt 18K resistor in each channel. I'll make some measurements when I get it running.
ab0ez
AB0EZ
One thing I really enjoy when rebuilding vintage amps is tossing that original, cracked and fire-starter power cord! I've put a nice 18ga 3-prong cord on this bad boy along with a ground lift switch. I removed the 120VAC accessory outlets, moved the power cord to one of those holes and put the ground lift switch where the cord used to go. Eico's are finicky about their grounding. This should enable my friend to select what works best in his particular setup and without altering the chassis!
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Jim -- A superior job indeed! A couple of points if I may:
1. The original heater supply had one side grounded. Since you are now using it to power the 12AX7, using some balancing resistors instead of the unbalanced approach will lower open input hum significantly.
2. It appears that you have doubled the value of the cathode bias resistors in the output stages. While I understand you may want to raise them slightly to help deal with any dissipation problems, raising them to such a high value will seriously compromise power output and distortion. I would go no higher than about 300 ohms max in each channel.
Your buddy is getting one heck of a rebuild job. Congrats on such excellent work!
Dave
PS: The 18K 1 watt resistors burn up because they are simply supplied as an underrated part in the stock design.
1. The original heater supply had one side grounded. Since you are now using it to power the 12AX7, using some balancing resistors instead of the unbalanced approach will lower open input hum significantly.
2. It appears that you have doubled the value of the cathode bias resistors in the output stages. While I understand you may want to raise them slightly to help deal with any dissipation problems, raising them to such a high value will seriously compromise power output and distortion. I would go no higher than about 300 ohms max in each channel.
Your buddy is getting one heck of a rebuild job. Congrats on such excellent work!
Dave
PS: The 18K 1 watt resistors burn up because they are simply supplied as an underrated part in the stock design.
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ab0ez
AB0EZ
Thanks Dave, high praise indeed!
If you take a look at one of the pictures where the 6.3VAC wires from the transformer hit the first EL34 socket, there's two 47 ohm resistors to ground, for a faux center tap. Kinda hard to see, but it's there.
Point taken on the cathode resistors. My friend was having to run this amp on a variac to keep the EL34s from going into volcanic red-plate mode. I used this value some time ago on another EL34 amp, so that was why I used them. I do also have a pair of 270, 300, 330 ohm 10 watt resistors and a couple of other values standing by to be subbed in once I figure out just where this amp is operating, bias wise.
And now for your enjoyment, attached is a snippet from page 6 of the Eico manual for the HF86. Did I mention Eico was happy to sell you replacement output tubes?
Thanks also for the information about the 18K resistors. I figured it was a case of building to a price point. Those 5 watt resistors I put in ought to outlive me!
If you take a look at one of the pictures where the 6.3VAC wires from the transformer hit the first EL34 socket, there's two 47 ohm resistors to ground, for a faux center tap. Kinda hard to see, but it's there.
Point taken on the cathode resistors. My friend was having to run this amp on a variac to keep the EL34s from going into volcanic red-plate mode. I used this value some time ago on another EL34 amp, so that was why I used them. I do also have a pair of 270, 300, 330 ohm 10 watt resistors and a couple of other values standing by to be subbed in once I figure out just where this amp is operating, bias wise.
And now for your enjoyment, attached is a snippet from page 6 of the Eico manual for the HF86. Did I mention Eico was happy to sell you replacement output tubes?
Thanks also for the information about the 18K resistors. I figured it was a case of building to a price point. Those 5 watt resistors I put in ought to outlive me!
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ab0ez
AB0EZ
Thank you Sir!
guitbanger
Active Member
Big feet.....big sound? lol
jaymanaa
RIP 1961-2018
Yep, size does matter. :yes:
ab0ez
AB0EZ
Thank you Sir! I'll be out of my area and in Yakistan (Yakima, WA) all this work week and I'll try and get a schematic drawn up that shows all the changes made to the amp. The chokes, BTW, are 50 ohm 1 Henry, same as in the Dynaco amps.
One of these days I'll get some Eico HF87 or 89 output transformers and make a pair of killer damper diode power supply monoblocks with fixed bias and a few other goodies under the hood!
One of these days I'll get some Eico HF87 or 89 output transformers and make a pair of killer damper diode power supply monoblocks with fixed bias and a few other goodies under the hood!
If you end up doing one for yourself, let me know. "Dr." Dave Gillespie modified the NFB circuit in a way that makes the amp much more stable -- and musical -- than the stock circuit. The mod can also be done with cathode bias.
I did a few other mods as well and I'll have to say the result ends amp "wanderlust" for me -- I'm completely satisfied with it.
I did a few other mods as well and I'll have to say the result ends amp "wanderlust" for me -- I'm completely satisfied with it.
just 
you
