Just started getting the finishing touches on a stereo 5881 project. This is a PPP UL amplifier with 5881 output tubes and 6SL7 in the driver section. The intended use of the amp is for an amplifier for my computer that is more convenient then the EZ10 mini-monoblocks that I built a while ago (The have been working on a daily basis for 6-8 hours).
The idea was to cram a pair of the EZ10's into one board. After looking around I found some schematics here and cobbled something together. While I'm not a fan of Cathode bias I used it here as a starter and for reasons of easy setup.
Here is what I have so far...
The idea was to cram a pair of the EZ10's into one board. After looking around I found some schematics here and cobbled something together. While I'm not a fan of Cathode bias I used it here as a starter and for reasons of easy setup.
Here is what I have so far...
Attachments
There appears to be a mistake on this schematic, the 25K balance pot should be connected to HV2, not the plate of the first 6SL7.
Good eye Kevin!
I was looking at this a few times and just didn't see it. Would have made for an odd sounding amp.
I looked at the PCB and it's just a very short jumper and a cutting of a trace and all good... or so I think 🙂
Thanks for the look at the schematic!
Sandy
I was looking at this a few times and just didn't see it. Would have made for an odd sounding amp.
I looked at the PCB and it's just a very short jumper and a cutting of a trace and all good... or so I think 🙂
Thanks for the look at the schematic!
Sandy
Looks like a more complicated Eico HF-87. Which is a nice sounding amp. What transformers are you using?
I need to be careful here not to shoot myself in the foot, but ...
if I read correctly there are other confusing things here: Firstly as Kevin said. On the pcb VR1 indeed seems to go in the plate circuit of the phase splitter (pin 5 of V2). But then C11 should have nothing to do with VR1, yet the top view of the pcb seems to show it directly going there as in the erroneous schematic. Also, C9 and C14 of the schematic seem to have been exchanged on the pcb. I have not investigated further and am not nit-picking, just to encourage the OP to investigate carefully before going on.
Then, SandyG, regarding cathode bias for the power stage: I read what your preference is, but keep in mind that by using a large cathode bypass capacitor (C12, C19), one can approximate fixed bias conditions for most music signals, because it doesn't change its potential very much if large enough. Not sure that the pcb can accomodate it, but with present-day small sizes, it is possible to fit in 1000µF or even 2200µF of the necessary voltage rating these days. Might as well try it while you have cathode bias ....
if I read correctly there are other confusing things here: Firstly as Kevin said. On the pcb VR1 indeed seems to go in the plate circuit of the phase splitter (pin 5 of V2). But then C11 should have nothing to do with VR1, yet the top view of the pcb seems to show it directly going there as in the erroneous schematic. Also, C9 and C14 of the schematic seem to have been exchanged on the pcb. I have not investigated further and am not nit-picking, just to encourage the OP to investigate carefully before going on.
Then, SandyG, regarding cathode bias for the power stage: I read what your preference is, but keep in mind that by using a large cathode bypass capacitor (C12, C19), one can approximate fixed bias conditions for most music signals, because it doesn't change its potential very much if large enough. Not sure that the pcb can accomodate it, but with present-day small sizes, it is possible to fit in 1000µF or even 2200µF of the necessary voltage rating these days. Might as well try it while you have cathode bias ....
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The PCB is definitely wrong 🙂 It does match the schematic, maybe I wasn't clear when I replied. I think the changes amount to cutting 2 traces and adding 2 short jumpers. I have not had a chance to update the schematics here yet but will hopefully get out to the workshop this afternoon and upload the new version.
For the cathode bypass caps (C12/C19) I left plenty of extra room in the component size for that part thinking ahead. I wanted to start out with what I had seen in the data sheets, but figured it was a place for leaving some extra room.
The question is what effect does the cathode bypass cap have as you make it larger to the sound or the circuits operation if anything other then stabilizing it.
The other thing I don't like about the cathode bias is the large space and heat for the resistors, I would much rather put in a few parts (including a nice glowing OB2 like the other amps I have done) and have easy adjustment of the bias. Space was tight so I thought I would give it a try again.
I did split up the cathode resistor into a pair of 5watt parts to allow for some more variety of values for bias setting and a bit easier to fit into the space then a single 10 watt part.
Will see how it all goes!
Thanks for the eyes on the project as well.
I have not seen the schematic for the Eico HF-87 but will take a look. The circuit came partly from some schematics here and the EZ10 project I did a while back which was a simple mini-monoblock. I decided to try to cram it all into a single board this time.
The intended use is a small tube amp for my computers.
I am using Edcor transformers for the project, I think the part numbers may be on the schematic on the first post.
Sandy
Wired up the amplifier last night. Other then wiring the ultra linear winding to backwards it was uneventful.
The amp ran about 21 watts into 8ohms before clipping, need to look at square wave at 10kHz as well as low frequency, but initial quick testing looked very good.
B+ with GZ34 rectifier and all tubes warmed up was about 397vdc, the bias was a tick on the hot side at 50ma for a near 20w on the plates (5881 max is 23w).
How does the amp sound?
Let's say I'm blown away how good this small amp works. When I first fired it up I was not expecting powerful bass, but totally wrong, I looked to see if the bass tone knob was twisted, but it wasn't. I played all my usual sources from classical to rockabilly and couldn't stop listening. I sat for about an hour and just enjoying music. The room I listened to it in is fairly large and with my old school Polk Audio speakers and sound level was not lacking. The amp had stunning high end with excellent clarity, no muddy mids or highs what so ever.
Will post some more data as I get it going.
Sandy
The amp ran about 21 watts into 8ohms before clipping, need to look at square wave at 10kHz as well as low frequency, but initial quick testing looked very good.
B+ with GZ34 rectifier and all tubes warmed up was about 397vdc, the bias was a tick on the hot side at 50ma for a near 20w on the plates (5881 max is 23w).
How does the amp sound?
Let's say I'm blown away how good this small amp works. When I first fired it up I was not expecting powerful bass, but totally wrong, I looked to see if the bass tone knob was twisted, but it wasn't. I played all my usual sources from classical to rockabilly and couldn't stop listening. I sat for about an hour and just enjoying music. The room I listened to it in is fairly large and with my old school Polk Audio speakers and sound level was not lacking. The amp had stunning high end with excellent clarity, no muddy mids or highs what so ever.
Will post some more data as I get it going.
Sandy
Spent a few minutes working on the amp the other day, swapped out the cathode bias resistors from 150ohm (total of 300) to 180ohm (total of 360). I think this will be a very good operating point for the 5881 at this voltage. Plate is right around 17w down from about 20w at idle (about 43ma at 404vdc).
Tossed on a square wave at 10khz and looked very square with a bit of ringing, no worse then anything else I have done, but doesn't mean it's good or bad. Will snap a picture of it.
Sandy
Tossed on a square wave at 10khz and looked very square with a bit of ringing, no worse then anything else I have done, but doesn't mean it's good or bad. Will snap a picture of it.
Sandy
Here are a couple of pics, the first one is the square wave at about 1/2 power (about 10 watts). Very clean except for the little glitch at the start of the waveforem.
The other 2 pics are the changes to the PCB that had to be made due to the error in the schematic. Noting major at all.
Also tossed out the GZ34 and put in one of the hacked up solid state regulators I made for the EZ125. With solid state rectification about 433 volts of B+ was had and the amp put out right at 25 watts both channels driven. On start up B+ was well over 450vdc (closer to about 480) until things were warmed up so I would not use this configuration without checking the bias again and replacing a couple of the caps that were 450v rated. This also might be pushing it for the smaller power transformer and choke used.
On to painting the crappy Hammond chassis...
Sandy
The other 2 pics are the changes to the PCB that had to be made due to the error in the schematic. Noting major at all.
Also tossed out the GZ34 and put in one of the hacked up solid state regulators I made for the EZ125. With solid state rectification about 433 volts of B+ was had and the amp put out right at 25 watts both channels driven. On start up B+ was well over 450vdc (closer to about 480) until things were warmed up so I would not use this configuration without checking the bias again and replacing a couple of the caps that were 450v rated. This also might be pushing it for the smaller power transformer and choke used.
On to painting the crappy Hammond chassis...
Sandy
Attachments
Here is the link to the finished project, plenty of picture in the gallery and the overly wordy descriptions 🙂
EZ215 Stereo Amplifier | EZ260
Sandy
EZ215 Stereo Amplifier | EZ260
Sandy
Would you have a power supply schematic with voltages? I building this amp with old Hammond organ xmers. I'm adding an input volume control for each channel. I'm very curious on how well the xmers sound or not?
I don't remember if I wrote them down other then the HV1 voltages at 404v with the GZ34 and biased for 17watts, and 433 with the solid state rectifiers. I will take a look and see if I put them on paper when I checked things out.
The Edcor's audio transformers on this amp (or the amp itself) sound very good. I was listening the other day and was still very impressed how it sounded.
I have a few PCB's left, PM me if interested.
Sandy
The Edcor's audio transformers on this amp (or the amp itself) sound very good. I was listening the other day and was still very impressed how it sounded.
I have a few PCB's left, PM me if interested.
Sandy
I didn't write down voltage with the final bias but here is what I had with the bias when the tubes were running about 20watts on the plate -
HV1 - 396
HV2, HV2 - 376
TP1 - 166
TP2 - 293
Hope this helps
Sandy
HV1 - 396
HV2, HV2 - 376
TP1 - 166
TP2 - 293
Hope this helps
Sandy
If you are in a mood to trim that, make C8 and c15 a little larger. Or ~ double the capacity with a small resistor ~1 Kohm in series with them.
I have a trimmer capacitor that I have used on another amp to adjust the value of the feedback cap, and was set to do it but after plugging it into some speakers and sitting down and listening I decided that I would not press my luck since it sounded so good to me.
I am building another one to experiment with and will mess with the feedback loop values a bit and see how it looks and sounds. Going to try some 6L6's to see how they work with a bit more voltage.
I don't know if the Tung-Sol 5881's are just 6L6's in a smaller bottle but will get a set of the Tung-Sol 6L6STR's and see if they are much different sounding and see how much they may churn out with a higher plate voltage. They are only a few bucks more and always good to have some 6L6 spares around.
Sandy
I am building another one to experiment with and will mess with the feedback loop values a bit and see how it looks and sounds. Going to try some 6L6's to see how they work with a bit more voltage.
I don't know if the Tung-Sol 5881's are just 6L6's in a smaller bottle but will get a set of the Tung-Sol 6L6STR's and see if they are much different sounding and see how much they may churn out with a higher plate voltage. They are only a few bucks more and always good to have some 6L6 spares around.
Sandy
The new Tung-Sol 6L6STR's came in today and I spent some time listening to them and then swapping out back to the 5881's. Very hard to hear the difference. The were both biased up to right about the same (43-44ma). The 6L6STR's bulb was a little bit longer and the guts of the tube were as well.
I don't expect any power difference, but I have not put it on the bench as yet or looked at the waveform.
I just finished putting together another board and waiting for a couple of transformers to try another configuration with the 6L6's and higher B+.
Sandy
I don't expect any power difference, but I have not put it on the bench as yet or looked at the waveform.
I just finished putting together another board and waiting for a couple of transformers to try another configuration with the 6L6's and higher B+.
Sandy
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