Chinese 807 amp mod suggestions

[wazza220951]

Bought a 807 off Ali Express, running Sylvania 807s, RCA 6SL7s and a Mullard 5V4G
Getting 355 volts at the anode. Sounds really good stock but any suggestions as to mods would be greatly appreciated.
Build quality is good and all txfmrs run cool.

 

[No Money]

I do like the hand drawn schematic.

Before the usual crowd pile on and tell you it's from China and is crap/was made by 8 year olds in a sweat shop etc. and you should throw it away I would usually suggest playing it for a while and trying some different tubes and coupling caps but assume from your post you have already upgraded the tubes. So maybe some better coupling caps?

 

[wazza220951]

I do like the hand drawn schematic.

Before the usual crowd pile on and tell you it's from China and is crap/was made by 8 year olds in a sweat shop etc. and you should throw it away I would usually suggest playing it for a while and trying some different tubes and coupling caps but assume from your post you have already upgraded the tubes. So maybe some better coupling caps?

Bought it without tubes as I have plenty of 807s and 6SL7s. Still burning it in so will give it a few more days.
I have plenty of russian PIOs I can try and will probably try LED bias.
I have bought a few amps from China, have never had a crap one yet as I do my homework first.

 

[No Money]

I have bought a few amps from China, have never had a crap one yet as I do my homework first.

That's the secret.

There's some good stuff out there if you look and do some basic research.

 

[wazza220951]

That's the secret.

There's some good stuff out there if you look and do some basic research.

Are you a kiwi mate?
Don't see many Raits on here!
Sold mine many moons ago and still regret it!

 

[trobbins]

You may want to use discrete, low-capacitance, shielded cable for the input channels - you may have cross-talk and high frequency loss. And possibly some shielding between the input sockets and the nearby speaker output wiring.

Double-check the ground wiring around the CLC B+ filter to ensure circuit grounds are taken from the second C neg terminal. Where is chassis grounded, and are the speaker outputs grounded?

What does the on/off switch switch? It looks like mains AC wiring passing nearby the 807 socket.

How does the 807 anode lead pass back through the chassis? The chassis doesn't use grommets, so hopefully that lead is suitable voltage rated, with extra insulation through the chassis.

 

[No Money]

Are you a kiwi mate?
Don't see many Raits on here!
Sold mine many moons ago and still regret it!

Well, I've been away for about 12 years, but yes, I supose I'm still a Kiwi. I still travel on that passport ...

Have had my Rait since 1977 or so, and it's currently back in play. A really great amp. Got it from a contact at the factory, if you can call the setup they had a factory ... Had it refreshed a while back and it should go for another 40 or so years.

Assume you are a transplant as well?

 

[wazza220951]

You may want to use discrete, low-capacitance, shielded cable for the input channels - you may have cross-talk and high frequency loss. And possibly some shielding between the input sockets and the nearby speaker output wiring.

Double-check the ground wiring around the CLC B+ filter to ensure circuit grounds are taken from the second C neg terminal. Where is chassis grounded, and are the speaker outputs grounded?

What does the on/off switch switch? It looks like mains AC wiring passing nearby the 807 socket.

How does the 807 anode lead pass back through the chassis? The chassis doesn't use grommets, so hopefully that lead is suitable voltage rated, with extra insulation through the chassis.

Have already done the grommets, The chassis wasn't grounded when I got it ! First thing I fixed. Changed the earthing at 2nd C neg at the same time.
One thing I have learnt when buying from overseas is to check all earthing to comply with Oz regulations and commonsense earthing practice. The on/off switch will be changed next to DP as it only switches L at the moment. Once the valves burn in will tidy things up. For what it cost though only minimal work needed to make it sing overall.

 

[wazza220951]

Well, I've been away for about 12 years, but yes, I supose I'm still a Kiwi. I still travel on that passport ...

Have had my Rait since 1977 or so, and it's currently back in play. A really great amp. Got it from a contact at the factory, if you can call the setup they had a factory ... Had it refreshed a while back and it should go for another 40 or so years.

Assume you are a transplant as well?

Yep, from Chch and have seen the "factory" so know what you mean. definitely a good amp.

 

[gadget73]

hm, no feedback anywhere. Bypassed cathode resistors too, so nothing local. Me being me I'd probably provide it with feedback, even if it was just removing the cathode caps to provide local feedback at each stage. The gain will drop considerably though, so it might require rewiring the 6SL7 to be 2 stages of gain vs a paralleled tube like it currently is.

of course a first step is a performance evaluation to see what its even doing now. Power output, frequency response, along with THD and IMD tests if you've got the gear.

 

[prelius]

There is bunch of changes you can make to this amp!!! I would do the following (my own sense of priority) changes:

1. Add local NF from the plate of 807 to the plates of the 6SN7/6SL7 (a 200K resistor)
2. Regulate voltage on G2 of the 807. This could be done in a number of ways, the most easy one using a resistor and a string of Zeners,totaling 300V (max voltage on G2).
3. Change the input stage from parallel plates to SRPP (I built both, and like SRPP better, YMMV)
3. Install a Triode/UL/Pentode Switch...

If you need schematics for any of the mods, please let me know, I will dig it out and post it.

Cheers, Paul.

 

[wazza220951]

There is bunch of changes you can make to this amp!!! I would do the following (my own sense of priority) changes:

1. Add local NF from the plate of 807 to the plates of the 6SN7/6SL7 (a 200K resistor)
2. Regulate voltage on G2 of the 807. This could be done in a number of ways, the most easy one using a resistor and a string of Zeners,totaling 300V (max voltage on G2).
3. Change the input stage from parallel plates to SRPP (I built both, and like SRPP better, YMMV)2
3. Install a Triode/UL/Pentode Switch...

If you need schematics for any of the mods, please let me know, I will dig it out and post it.

Cheers, Paul.

Thanks Paul, a schematic for the SRPP would be great.Will check the voltage on G2, possibly a bit high now I have dropped a 5V4G in there. Might try LED bias on the 6SL7 as well but will try your suggestions first.
Will definitely start with the local NF

 

[wazza220951]

hm, no feedback anywhere. Bypassed cathode resistors too, so nothing local. Me being me I'd probably provide it with feedback, even if it was just removing the cathode caps to provide local feedback at each stage. The gain will drop considerably though, so it might require rewiring the 6SL7 to be 2 stages of gain vs a paralleled tube like it currently is.

of course a first step is a performance evaluation to see what its even doing now. Power output, frequency response, along with THD and IMD tests if you've got the gear.

Unfortunately the only test gear I have are my ears and a multimeter, might be time to invest in a scope.

 

[mjw21a]

I have bought a few amps from China, have never had a crap one yet as I do my homework first.

My experience also with the best I've tried so far being my Oldchen K3. Incredible with Tung Sol KT120, 2x Shuguang CV181-Z Treasures and 1x Melz 6H8C hole plate tubes, coupling cap and volume pot upgrades. Will have it switchable to triode eventually too

 

[wazza220951]

My experience also with the best I've tried so far being my Oldchen K3. Incredible with Tung Sol KT120, 2x Shuguang CV181-Z Treasures and 1x Melz 6H8C hole plate tubes, coupling cap and volume pot upgrades. Will have it switchable to triode eventually too

View attachment 1021383[

My experience also with the best I've tried so far being my Oldchen K3. Incredible with Tung Sol KT120, 2x Shuguang CV181-Z Treasures and 1x Melz 6H8C hole plate tubes, coupling cap and volume pot upgrades. Will have it switchable to triode eventually too

View attachment 1021383

Yep the only problems I have had has been electrical earth not grounded to chassis. A common problem with overseas amps and the first thing I check.

 

[arts]

Unfortunately the only test gear I have are my ears and a multimeter, might be time to invest in a scope.

Frankly Wazza,a scope is probably the least useful piece of test equipment in the audio domain. Other than showing outright clipping,really not much to be had there.

While I do not have any knowledge of the Australian used test equipment situation,you would undoubtably be better off with a dedicated audio analyzer.

With their own internal signal generators,which are usually very closely matched in performance to their own internal distortion analyzers,and the ability to very accurately measure voltages regardless of frequency,you are getting the ''complete package'', as it were.Most can also run frequency response & distortion vs frequency sweeps,noise level tests etc. Many will also output a GPIB signal which allows you to interface with your computer,allowing you to capture screen plots which you can save or print,and also control the machine via the keyboard. When you consider the price of separate instruments,the inherent difficulties of matching/interfacing,and the amount of bench space required for separates,analyzers are a pretty sweet proposition Keithley makes a model that is available inexpensively on the used market,but unfortunately the generator falls short of the analyzers capability.This is the one I use,just before I installed it on my bench:

 

[gadget73]

Short of that, a good accurate meter that will do up to 20khz and a signal generator can get you freqency response plots. Its tedious, but it can be done. I'm now down the meter, so its back to the analog in the THD analyzer for voltage readings now. On my list of things to do is upgrade my test setup, maybe actually end up with something automated so its not 20 minutes of pushing buttons and recording results in a spreadsheet, then having it math it and graph it for me every time I change something and want to see what it did. Its just tedious.

 

[prelius]

Attached to this post is an example of 807 with SRPP front end. The schematics is in Russian, but no translation is needed, I believe.
Cheers, Paul.

 

[wazza220951]

Attached to this post is an example of 807 with SRPP front end. The schematics is in Russian, but no translation is needed, I believe.
Cheers, Paul.

Thanks Paul, just checked out your FB page, hmmm I have a bunch of Gu50s I picked up cheap about a decade ago, I can see another project coming up!
I will try the other mods first b4 going srpp and see how it sounds.

 

[wazza220951]

Short of that, a good accurate meter that will do up to 20khz and a signal generator can get you freqency response plots. Its tedious, but it can be done. I'm now down the meter, so its back to the analog in the THD analyzer for voltage readings now. On my list of things to do is upgrade my test setup, maybe actually end up with something automated so its not 20 minutes of pushing buttons and recording results in a spreadsheet, then having it math it and graph it for me every time I change something and want to see what it did. Its just tedious.

Thanks, I will check out the used market here and see what I can find