goldear
Certifiable Audio Junkie
This is a project that I have been planning on doing for literally years now. But that I finally completed recapping this unit over Christmas. And I have been rolling tubes over the past couple of weeks in an attempt to find the perfect combination of tubes to really make this amp sing.
The intention of this rebuild was to make the amp stable again, and improve is sonics a bit, but without completely changing the beautiful sound characteristics of this classic old amp. So for the initial part of this project has been confined to recapping for the most part, with only very minor upgrades being implemented while I was at it.
The circuit is still stock (or is at least in the same form which I found it in) with the exception of some very minor upgrades, including a Hexfred rectifiers, adding some 100 Ohm stabilizing resistors to the screens, and increasing the size of all of the power supply reservoir caps.
I replaced all of the old paper and ceramic caps with polystyrene caps. I avoided polypropylene's because I think that this amp is a bit too-bright sounding to have any added brightness which seems be present in many polypropylene capacitors. Mostly these were Multicap RTX, with the exception of the pF value caps, which I just used small styrenes from my stash of caps. The only exception to this was the "death cap" which I replaced with a safety-type cap.
For the two cathode bypass caps I installed a couple of black-gates from my cap-stash. To replace the filter cans I used a bunch of JJ caps. I was too lazy to re-stuff the original cans.
http://sanacacio.net/hifi/hf89schematic.jpg
However in the course of my work, I discovered that my unit is not quite the same as the schematic which I am referencing above. I know that there were two versions of this amp, but I don't know which version mine is.
The differences that I found were that the main feedback loop had a 175pF cap in parallel with a single 6k resistor. There was/is no 500pF cap going to the 4 ohm tap. Nor is there a 750pF Cap in parallel with half of the transformer primary winding. Plus there was one additional 15pF cap which is nowhere on this schematic which appears to be used for some inter-stage feedback between one of the outputs and one of the drivers. The other big difference is that mine has a 100 Ohm resistor feeding the output-stage filter cap, not the 50 Ohm resistor shown in the schematic. So I would really like to know if mine is original, or the revised version of this amp.
Prior to the rebuild, it was not possible to keep output tubes from going cherry for very long, no matter how low I tried to bias them. Now the outputs appear to be dead stable, and the bias is holding very stable. I'm not sure how much of this is due to the addition of the 100 Ohm screen resistors, vs the new caps. But whatever the case, I love the fact that this amp is finally stable with any EL 34 I have tried in it.
I've not done anything special to reduce the input voltage yet, other than running it off of my variac for the moment. I'm not sure how necessary this really is though, especially since this already has a 100 Ohm resistor prior to output caps. But for the moment, just to be safe, I'm running it at about 115 VAC.
Tube rolling has been interesting. I've played with all of the following:
Mullard EL34s RCA Fat-bottle EL-34s, EH EL34s, EH 6CA7s, Svetlana EL34s (not the winged Cs), and JJ KT77s.
So far I'm torn between the sound of the Svetlana El34s and the JJ KT77s, preferring these over all of the others I have tried so far. I'm also using a 5751 on the input which I prefer over the original mullard 12AX7.
I'd really like to try some winged-Cs, and some of the GEC KT77s, and the new mullards, and new Tung Sols too. But unfortunately my only set of Winged-Cs had a single defective tube, so I have yet to try any of these. But it gets expensive trying tubes, so perhaps I'll just quit while I'm ahead.
So far this amp is sounding very nice. It has a huge sound-stage, gorgeous tube mids, and highs. It's not a particularly detailed-sounding amp so far, but it is oh so musical! I'm not sure if it is my imagination or not, but the sound seems to be improving gradually as the new caps have burned-in a bit.
I've not decided yet if I'm going to try to improve the detail a bit by upgrading the resistors or not. But I probably will at least add some power supply bypass caps. Anyways so far, so good.
I'll try to post some pictures a little later.
The intention of this rebuild was to make the amp stable again, and improve is sonics a bit, but without completely changing the beautiful sound characteristics of this classic old amp. So for the initial part of this project has been confined to recapping for the most part, with only very minor upgrades being implemented while I was at it.
The circuit is still stock (or is at least in the same form which I found it in) with the exception of some very minor upgrades, including a Hexfred rectifiers, adding some 100 Ohm stabilizing resistors to the screens, and increasing the size of all of the power supply reservoir caps.
I replaced all of the old paper and ceramic caps with polystyrene caps. I avoided polypropylene's because I think that this amp is a bit too-bright sounding to have any added brightness which seems be present in many polypropylene capacitors. Mostly these were Multicap RTX, with the exception of the pF value caps, which I just used small styrenes from my stash of caps. The only exception to this was the "death cap" which I replaced with a safety-type cap.
For the two cathode bypass caps I installed a couple of black-gates from my cap-stash. To replace the filter cans I used a bunch of JJ caps. I was too lazy to re-stuff the original cans.
http://sanacacio.net/hifi/hf89schematic.jpg
However in the course of my work, I discovered that my unit is not quite the same as the schematic which I am referencing above. I know that there were two versions of this amp, but I don't know which version mine is.
The differences that I found were that the main feedback loop had a 175pF cap in parallel with a single 6k resistor. There was/is no 500pF cap going to the 4 ohm tap. Nor is there a 750pF Cap in parallel with half of the transformer primary winding. Plus there was one additional 15pF cap which is nowhere on this schematic which appears to be used for some inter-stage feedback between one of the outputs and one of the drivers. The other big difference is that mine has a 100 Ohm resistor feeding the output-stage filter cap, not the 50 Ohm resistor shown in the schematic. So I would really like to know if mine is original, or the revised version of this amp.
Prior to the rebuild, it was not possible to keep output tubes from going cherry for very long, no matter how low I tried to bias them. Now the outputs appear to be dead stable, and the bias is holding very stable. I'm not sure how much of this is due to the addition of the 100 Ohm screen resistors, vs the new caps. But whatever the case, I love the fact that this amp is finally stable with any EL 34 I have tried in it.
I've not done anything special to reduce the input voltage yet, other than running it off of my variac for the moment. I'm not sure how necessary this really is though, especially since this already has a 100 Ohm resistor prior to output caps. But for the moment, just to be safe, I'm running it at about 115 VAC.
Tube rolling has been interesting. I've played with all of the following:
Mullard EL34s RCA Fat-bottle EL-34s, EH EL34s, EH 6CA7s, Svetlana EL34s (not the winged Cs), and JJ KT77s.
So far I'm torn between the sound of the Svetlana El34s and the JJ KT77s, preferring these over all of the others I have tried so far. I'm also using a 5751 on the input which I prefer over the original mullard 12AX7.
I'd really like to try some winged-Cs, and some of the GEC KT77s, and the new mullards, and new Tung Sols too. But unfortunately my only set of Winged-Cs had a single defective tube, so I have yet to try any of these. But it gets expensive trying tubes, so perhaps I'll just quit while I'm ahead.
So far this amp is sounding very nice. It has a huge sound-stage, gorgeous tube mids, and highs. It's not a particularly detailed-sounding amp so far, but it is oh so musical! I'm not sure if it is my imagination or not, but the sound seems to be improving gradually as the new caps have burned-in a bit.
I've not decided yet if I'm going to try to improve the detail a bit by upgrading the resistors or not. But I probably will at least add some power supply bypass caps. Anyways so far, so good.
I'll try to post some pictures a little later.
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