L.S.,
I just bought myself a Audio Innovations L1 and a AI 800MkIII. I want to measure all myself and also set the bias for the four EL34/6ac7 tubes. Who can help me to do it. If there is a way to send pictures or a video, that would be most welcome!
Ton from the Netherlands
I just bought myself a Audio Innovations L1 and a AI 800MkIII. I want to measure all myself and also set the bias for the four EL34/6ac7 tubes. Who can help me to do it. If there is a way to send pictures or a video, that would be most welcome!
Ton from the Netherlands
Dear JonSnell,
Thanks for your reply. This is the second time this evening I got that answer. I spoke mr. Ed de Jong from Dejongsystems in the Netherlands, an expert too in Audio Innovations. He told me the same. When I have a matched quad EL34s I can put them in and play!
That remind me of another question: was the 800 designed only for the EL34 tubes or are other tubes also possible?
Kind regards,
Ton
Thanks for your reply. This is the second time this evening I got that answer. I spoke mr. Ed de Jong from Dejongsystems in the Netherlands, an expert too in Audio Innovations. He told me the same. When I have a matched quad EL34s I can put them in and play!
That remind me of another question: was the 800 designed only for the EL34 tubes or are other tubes also possible?
Kind regards,
Ton
Hi, My amp was an MK-1 so I don't know how relevnt my comments are compared to the Mk-3. You can find a picture and scematics of an amp similar to mine here: Audio Innovations Series 800 mkI power amp Photo #233823 - US Audio Mart
If yours is same/simmilar I can post some pictures of the internals tomorrow.
On my AI-800 the Cathode resistors were all over the place in value so in my case there were not much use in matching tubes. I had to add series resistors on some of the cathode-resistors an shunted others to make them all same value. Not much reason for matching tubes until this was done.
Also, some of the electrolytic had developed DC resistance- possibly due to tube failure, eventually leading to capacitor exploding.
Also note that if you intend to open your AI-800, mine did not have any bleeder connected to the reservoir capacitor. The capacitors can store 400+ volts for days so make sure the first thing you do is to discharge those. I ended up soldering 470K resistor across one of the the caps.
As for alternative tubes, the old BorderPatrol web site used to recomend russian made 5881 tubes for longevity (Sovtek?). I was using 6L6GC on mine. Other than than I guess KT77 and or 6CA7 or close equvivalents to EL34, but have not heard of anyone using them in this amp.
My amp had seen a very rough life before I purchased it for the transformers.
EL34 Internal shorts
With the amp off, and the caps discharged:
Verify that all EL34 tubes have their own self bias resistor.
Then measure from pin 8 of each EL34 to ground, the resistances should all be the same.
If it is only a very low resistance, like 10 ohms or so, then fixed bias is being used.
Otherwise, with a few hundred ohms, self bias is being used.
If self bias is being used, power up the amp (be sure to load both L and R outputs).
Measure the voltage of each EL34 cathode. The voltages (a few 10s of volts) should be close to each other, or else you need a set of matched tubes.
If fixed bias is used, measure the EL34 cathode voltages. Example, 35mA of current in a 10 Ohm resistor is 0.35V. The voltages should all be close . . .
Or you have individual bias pots for each tube, and must adjust them . . .
Or you need matched EL34 tubes if there are no individual adjustments for each tube.
The KT77 is the closest replacement for the EL34. The specifications are almost identical.
The major difference is in the construction: KT77 is a Beam Power tube, EL34 is a Pentode tube.
The other difference is that the suppressor on the EL34 is pin 1, and must be connected to pin 8 Cathode.
The KT77 Beam formers are connected internally to the cathode. Pin 1 can be left unconnected, or can be connected to pin 8 with out any trouble.
Do not use a 6550 as an EL34 substitute (you will get the 6550 going into thermal run-away, and destroy one or more parts.
6550 in fixed bias needs 50k or less grid resistor, and 6550 in self bias needs 100k or less grid resistor.
Your amp has grid resistors of 220k ohms to ground (according to the schematic JonSnell provided).
Check that those resistors are 220k, or what value are they?
A KT88 in self bias mode will work with a grid resistor of up to 220k,
But with fixed bias will only work with a grid resistor of up to 100k.
There are some other less stringent KT88 grid resistor specs, but only at lower plate and screen dissipations.
You would have to check the plate voltages, screen voltages, and cathode currents to see what the dissipation is.
You should be able to use KT88 tubes, but only within the restrictions, especially related to the grid resistors versus bias (self or fixed).
Verify that all EL34 tubes have their own self bias resistor.
Then measure from pin 8 of each EL34 to ground, the resistances should all be the same.
If it is only a very low resistance, like 10 ohms or so, then fixed bias is being used.
Otherwise, with a few hundred ohms, self bias is being used.
If self bias is being used, power up the amp (be sure to load both L and R outputs).
Measure the voltage of each EL34 cathode. The voltages (a few 10s of volts) should be close to each other, or else you need a set of matched tubes.
If fixed bias is used, measure the EL34 cathode voltages. Example, 35mA of current in a 10 Ohm resistor is 0.35V. The voltages should all be close . . .
Or you have individual bias pots for each tube, and must adjust them . . .
Or you need matched EL34 tubes if there are no individual adjustments for each tube.
The KT77 is the closest replacement for the EL34. The specifications are almost identical.
The major difference is in the construction: KT77 is a Beam Power tube, EL34 is a Pentode tube.
The other difference is that the suppressor on the EL34 is pin 1, and must be connected to pin 8 Cathode.
The KT77 Beam formers are connected internally to the cathode. Pin 1 can be left unconnected, or can be connected to pin 8 with out any trouble.
Do not use a 6550 as an EL34 substitute (you will get the 6550 going into thermal run-away, and destroy one or more parts.
6550 in fixed bias needs 50k or less grid resistor, and 6550 in self bias needs 100k or less grid resistor.
Your amp has grid resistors of 220k ohms to ground (according to the schematic JonSnell provided).
Check that those resistors are 220k, or what value are they?
A KT88 in self bias mode will work with a grid resistor of up to 220k,
But with fixed bias will only work with a grid resistor of up to 100k.
There are some other less stringent KT88 grid resistor specs, but only at lower plate and screen dissipations.
You would have to check the plate voltages, screen voltages, and cathode currents to see what the dissipation is.
You should be able to use KT88 tubes, but only within the restrictions, especially related to the grid resistors versus bias (self or fixed).
Last edited:
6A3sUMMER, LJT and JonSnell,
Thanks for your suggestions and advices. I’m new here on diyaudio and a novice in tubeland.
I’ve always had solid state amplifiers but I am curious to tubegear and my intention is to build a tube amplifier myself. So all help is welcome! Therefore, thank you for your reactions and replies. It feels like walking on the sunny side of the street.
Thanks for your suggestions and advices. I’m new here on diyaudio and a novice in tubeland.
I’ve always had solid state amplifiers but I am curious to tubegear and my intention is to build a tube amplifier myself. So all help is welcome! Therefore, thank you for your reactions and replies. It feels like walking on the sunny side of the street.
To LJT
2015
Quote:
Originally Posted by tmnn1711 View Post
.......... When I have a matched quad EL34s I can put them in and play!
That remind me of another question: was the 800 designed only for the EL34 tubes or are other tubes also possible?
..........
Hi, My amp was an MK-1 so I don't know how relevnt my comments are compared to the Mk-3. You can find a picture and scematics of an amp similar to mine here: Audio Innovations Series 800 mkI power amp Photo #233823 - US Audio Mart
If yours is same/simmilar I can post some pictures of the internals tomorrow.
Between the 800 MkI en Mk III are similarities and differences. They are not similar, but I’m always interested in some pictures. If I’ll open my amplifier I’ll remember your advice! and put one hand behind mij back! 😉
Thanks for your suggestions
Ton
2015
Quote:
Originally Posted by tmnn1711 View Post
.......... When I have a matched quad EL34s I can put them in and play!
That remind me of another question: was the 800 designed only for the EL34 tubes or are other tubes also possible?
..........
Hi, My amp was an MK-1 so I don't know how relevnt my comments are compared to the Mk-3. You can find a picture and scematics of an amp similar to mine here: Audio Innovations Series 800 mkI power amp Photo #233823 - US Audio Mart
If yours is same/simmilar I can post some pictures of the internals tomorrow.
Between the 800 MkI en Mk III are similarities and differences. They are not similar, but I’m always interested in some pictures. If I’ll open my amplifier I’ll remember your advice! and put one hand behind mij back! 😉
Thanks for your suggestions
Ton
To 6A3sUMMER,
thanks for your comprehensive and educational answer. I can learn from that. I understand that my AI 800 Mk3 has a cathode bias or selfbias.
Your suggestions for other tubes are very useful for the future. I’ve stil a lot to learn but I love experimenting! Thanks
Ton
thanks for your comprehensive and educational answer. I can learn from that. I understand that my AI 800 Mk3 has a cathode bias or selfbias.
Your suggestions for other tubes are very useful for the future. I’ve stil a lot to learn but I love experimenting! Thanks
Ton