Vacuum Tube OTL power amp!!

M Gregg · 2011-12-11 2:18 pm

Looking forward to your progress..

I like wood..LOL

Just an interesting picture...

Modified Futterman OTL amplifier - cellai@space.it - OTL Asylum

Reflecting on the picture the 6080 is a tube I had not thought about...

Go on lash it up for fun and have a listen...Then you can assess how hot its going to get and do any tweeks before vacuum packing it...

Regards
M. Gregg

flatheadmurre · 2011-12-11 3:07 pm

6AS7/6080 is a good one, the amp i sold to pay for this was a PP 6080.

Somewhere in the darkest places of storage i have loads of new PL36 from the TV-era something for another OTL.

Have you tried to hook it up to 16 or 32ohm load and measured it ?

Tells a lot about the outputstage.

M Gregg · 2011-12-11 3:43 pm

flatheadmurre said:
6AS7/6080 is a good one, the amp i sold to pay for this was a PP 6080.

Somewhere in the darkest places of storage i have loads of new PL36 from the TV-era something for another OTL.

Have you tried to hook it up to 16 or 32ohm load and measured it ?

Tells a lot about the outputstage.

I was just looking at this schematic

6AS7GA OTL

They have achieved the drive voltage swing by using 550V on the driver stage..

just wondering about the Mini OTL idea it would have to be fixed bias to get the power from a small size... Mini and power efficient<<< The green OTL LMAO I guess OTL and Green don't go togeather..

I am waiting for your test results ... I don't want to take the fun out of it...LOL

Its been on for 6Hrs so far today..

..sounds great!

Regards
M. Gregg

tubemax · 2011-12-12 8:20 pm

M Gregg said:
I was just looking at this schematic

6AS7GA OTL

They have achieved the drive voltage swing by using 550V on the driver stage..

Hi M. Gregg, I made this amplifier ( from Glass Audio ) more than 10 years ago - it sounds superb!
This was my first-hand experience with OTL amplifiers > it sounded way better than good 300B SET amp side-by-side... with zero problems regarding dc shift in output stage...

Max

flatheadmurre · 2011-12-12 8:49 pm

Amp up and running for about 5 min then the filament reg blew :bawling:

Murphys law OTL amp strikes back again

M Gregg · 2011-12-12 10:19 pm

flatheadmurre said:
Amp up and running for about 5 min then the filament reg blew

Murphys law OTL amp strikes back again

Try a 3/16 pin in place of the regulator.... it has better current capacity..

Regards
M. Gregg

M Gregg · 2011-12-12 10:27 pm

tubemax said:
Hi M. Gregg, I made this amplifier ( from Glass Audio ) more than 10 years ago - it sounds superb!
This was my first-hand experience with OTL amplifiers > it sounded way better than good 300B SET amp side-by-side... with zero problems regarding dc shift in output stage...

Max

Thank's for the info..

Looks interesting... what was the heat like from the Op tubes?

Regards
M. Gregg

tubemax · 2011-12-13 10:58 am

It wasn't that bad, surely more heat than average tube amp but still very tolerable in the small room.

Max

M Gregg · 2011-12-13 1:17 pm

tubemax said:
It wasn't that bad, surely more heat than average tube amp but still very tolerable in the small room.

Max

Max,

Do you still have the amp?

What happens if you use only 2 or 4 X 6080 in each channel
(is the OP impedance to high ie not able to drive the speaker) wattage level ?

Just looking at a few ideas...

Regards
M. Gregg

tubemax · 2011-12-13 3:12 pm

Hi M. Gregg, I sold that amp years ago in pursuit of even "better" tube amplifiers but it was capital mistake... ( I see that now )
This amp was build as a stereo version with 4x6as7 per channel & at that time I was using Rogers LS3/5a speakers ( 11 ohm ) - in spite of 82.5db sensitivity amp would drive this speakers with ease...
I never used amp with less than 4 tubes per channel because of filament power supply was wired in series per 4 tubes ( about 25V DC )...
I'm busy at the moment with one OTL amp with 6080 tubes, when I have working prototype - I'll let know everybody about it...

Regards,
Max

M Gregg · 2011-12-14 6:56 pm

flatheadmurre said:
Amp up and running for about 5 min then the filament reg blew

Murphys law OTL amp strikes back again

Any news?

I guess you have solved the Reg problem?

Regards
M. Gregg

flatheadmurre · 2011-12-15 9:40 pm

One of the 6J5s has a bad filament, very hungry for current

Have to go storage diving for a new in the weekend.

M Gregg · 2011-12-16 8:15 am

flatheadmurre said:
One of the 6J5s has a bad filament, very hungry for current

Have to go storage diving for a new in the weekend.

I just used 7812 with a diode in the leg to lift it to 12.6 then series the heaters...easy overcurrent protection float the ov rail with a 0.1 polypropylene <<< made a big difference in hum reduction.

Regards
M. Gregg

M Gregg · 2011-12-16 8:17 am

Did it damage the regulator?

I guess you could have used a Maida type circuit better protection and used it based on higher current...

flatheadmurre · 2011-12-16 8:24 am

The problem when i get some time waiting for components is that the rest get overenginered.

Back to KISS and drive by enginering

I have one 6.3 feed for small signal heater.

MbR735 bridge 20mF 1084.

I suspect some kind of short between HT and filament otherwise the overload protection should save the 1084.

M Gregg · 2011-12-16 8:29 am

Just for interest,

I have received the CC 330 Ohm resistors, Just waiting for some orange drop 716 to try for fun in the o/p from the PI to 6c33c before I pull it to bits... I will put some PTFE sleeving over the resistor leg before fitting the ferrite bead. Reason is I have found ferrite can smear the sound when in direct contact with the conductor.

Regards
M. Gregg

M Gregg · 2011-12-16 8:32 am

flatheadmurre said:
The problem when i get some time waiting for components is that the rest get overenginered.

Back to KISS and drive by enginering

I have one 6.3 feed for small signal heater.

MbR735 bridge 20mF 1084.

I suspect some kind of short between HT and filament otherwise the overload protection should save the 1084.

I use DC for the input tubes and AC for the PI... How did you get it into the box? I know put it in the food blender then pour it in the box...

flatheadmurre · 2011-12-16 8:34 am

If i run the 6j5 on ac with the existing voltage ref and only 5692 with dc floating heater.

Have you figured out why it has less hum floating ?

6SN7 usually likes elevated heaters around 50v.

M Gregg · 2011-12-16 8:45 am

flatheadmurre said:
If i run the 6j5 on ac with the existing voltage ref and only 5692 with dc floating heater.

Have you figured out why it has less hum floating ?

6SN7 usually likes elevated heaters around 50v.

The only thing I can think of, is there can be no difference between heater and cathode voltage because the cap will charge to what ever value it needs to be equal..
I don't know why but with DC and a fixed lift voltage the hum would vary as the tube got hotter with use.. It also created pick up on the grid from the volume pot. With the heaters floating with a cap all this pick up stopped..

I found that when the 6c33cs were pulled out of circuit I could hear any noise from the pre section in the speakers. So its interesting to listen to the OP with speakers connected and no power tubes just to try it..
(the PI return to supply creates this)

M Gregg · 2011-12-16 8:49 am

Have you worked out a way to get everything in the box?

The heat is a right pain in the a.. LOL

Vacuum Tube OTL power amp!!

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