I think it said V3-V17 and V18-V32 on the drawing so it's a simplified schematics for output tubes.
First Power-up in 55 Years - Need Advice
- Thread starter Kasanay
- Start date
I think it said V3-V17 and V18-V32 on the drawing so it's a simplified schematics for output tubes.
Kasanay
Active Member
Yes, the schematic only shows 4 of the 30 required tubes. All tubes are wired in parallel.
If anybody remembers that horror-show that was published in Glass Audio long ago,you'll know what I'm talking about!
trobbins
Super Member
Perhaps fuse each transformer primary would be a good idea, given there are 3 transformers.
That 6AU6 is certainly working at a low current level, so hopefully its not too noisy. There are lots of higher frequency response roll-off caps in the forward path.
The bias take-offs could probably do with some decoupling if there is some hum present in the output.
It looks like the output signal return current path includes the shielded cable - does that connect directly to the 'G' star point?
What do the main B+/- rails operate at? There may be some benefit in biasing the heater CTs to +/- levels to reduce the heater-cathode stresses.
That 6AU6 is certainly working at a low current level, so hopefully its not too noisy. There are lots of higher frequency response roll-off caps in the forward path.
The bias take-offs could probably do with some decoupling if there is some hum present in the output.
It looks like the output signal return current path includes the shielded cable - does that connect directly to the 'G' star point?
What do the main B+/- rails operate at? There may be some benefit in biasing the heater CTs to +/- levels to reduce the heater-cathode stresses.
Now I'm curious. I know pretty little about OTL but the handful I've seen were mostly large parallel arrangements. I could swear I remember one that was a massive cathode follower rig, which seems sensible for lower output impedance. I have a Phillips designed radio with that sort of thing, EL84 plate driving an EL86 grid, output coupling capacitor off the EL86 cathode into 800 ohm speakers.
Kasanay
Active Member
I have read about that Phillips approach. It was an attempt to reduce the production cost of the radio (by eliminating the OPT). Stevens also did this with a 500 ohm amplifier that they paired up with a Stevens 500 ohm speaker.
My OTLs are intended to drive loads no less than 16 ohms and should be happy with 32 ohm speakers. I happen to own the 32 ohm speakers the belong to this rig - a nice pair of custom Bozaks.
Kasanay
Active Member
trobbins,
Thanks for you comments.
The transformers are protected with a common 10A fuse. Are you saying I should look at providing a dedicated fuse for each transformer?
I am not yet to a point where I can check the noise level from the 6AU6. I have only run this for short lengths of time. I need to do the re-cap.
Can you elaborate your comment about the "higher frequency response roll-off caps in the forward path"? I am still green at this and would appreciate further clarification.
I will keep it mind about the possible need to decouple the bias take off. If I get any hum, I'll look into this (and maybe contact you for more thoughts).
The shielding is connected to the G star point right at the input jack. This is also where the G buss is connected to chassis ground.
I'm not sure about the B+ rail voltage as I have not measured this. I have checked the points listed in the Voltage Chart and found all of them to be surprising close to spec.
Thanks,
s-petersen
Scott
Personally I would leave it the stock design,and just change out the aged caps and out of spec parts.The amp may need some roll off as there is no real limit on frequency response.C16 18 19 20 all are designed to cut some high frequencies and prevent oscillations
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Kasanay
Active Member
That is my plan. I do not know that I need to "improve" anything but I am collecting thoughts and ideas at this time. I am comfortable with changing the caps and things on that level but when you say something like: "The amp may need some roll off as there is no real limit on frequency response." I need a little hand-holding. I assume this means adding some caps, somewhere, but I would need some guidance.
Thanks
s-petersen
Scott
The amp without any roll off may oscillate at RF frequencies and become a radio transmitter, that is why I would not change the value of the caps and resistors in the circuit.The amp is already likely designed with the proper roll off as is.
Possibly you could contact Mathew Polk and find out about his experience with mods and reliability if his are in regular use.
Possibly you could contact Mathew Polk and find out about his experience with mods and reliability if his are in regular use.
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Kasanay
Active Member
That's a great idea. However, I will save that call for later. I do have one other source for first-hand knowledge of these amps and will be contacting that gentelman soon. Right now I will be swapping out the electrolytic caps while preserving the values and the appearance.
Thanks,
trobbins
Super Member
Yes fusing each transformer separately should provide better opportunity for the fuse to blow before some circuit parts fail from over-current - I doubt you would find off the shelf transformers for that amp
Each stage of an amps circuitry includes a high pass filter - usually the amp designer makes one stage have a dominantly low roll-off frequency, or else the output transformer (in a normal amp) becomes the part that rolls off the frequency (that gets tricky then - as exampled by the Williamson design). Such a dominant roll-off frequency then allows the designer to add a feedback path and for the closed loop to not be unstable. Assessing all the stages and working out the appropriate feedback can get quite tricky, especially where the roll off of a stage is the valve internal resistance followed by the miller capacitance of the next stage - which forms an RC low pass filter. All those caps C16,18,19,20 add extra (stable) capacitance to the somewhat variable miller capacitance of the next stage. Even though the amp was designed as stable, you should check for high frequency performance using square waves - as that is often the easiest stability check to make.
Kasanay
Active Member
Yes fusing each transformer separately should provide better opportunity for the fuse to blow before some circuit parts fail from over-current - I doubt you would find off the shelf transformers for that amp
Each stage of an amps circuitry includes a high pass filter - usually the amp designer makes one stage have a dominantly low roll-off frequency, or else the output transformer (in a normal amp) becomes the part that rolls off the frequency (that gets tricky then - as exampled by the Williamson design). Such a dominant roll-off frequency then allows the designer to add a feedback path and for the closed loop to not be unstable. Assessing all the stages and working out the appropriate feedback can get quite tricky, especially where the roll off of a stage is the valve internal resistance followed by the miller capacitance of the next stage - which forms an RC low pass filter. All those caps C16,18,19,20 add extra (stable) capacitance to the somewhat variable miller capacitance of the next stage. Even though the amp was designed as stable, you should check for high frequency performance using square waves - as that is often the easiest stability check to make.
Thanks for taking time to explain this so well. I will look into adding fuses. One challenge is doing it without changing the appearance of the unit. I'll need to hide the fuse holders somehow.
C16, C18, C19 & C20 are all mica caps and it my understanding these very rarely fail. I do not plan to replace them - at least not initially. After I get the re-cap job completed, I'll be looking at more of the performance details as you suggest.
Thanks,
Ginger!
Kasanay
Active Member
I like it.
Jim Ackerly
New Member
!! This is the first time I have seen any reference to an MP Audio system on the Net. I was the executor of Mirko Paneyko's estate and have many papers of his, as well as the remains of his "A" studio system. Where and when did you get that console system and those magnificent speakers?
Kasanay
Active Member
Hello Jim,
I would love to talk to you about Mirko.
I sent a conversation request to to you. Please check that.
Joe Y.
jdsalinger
Super Member
Man, that demo video....the amp sounds GREAT! (even played on my JBL TLX's...and I don't even LIKE RHCP)
I'd like more info on this amp/console too. Do you know the history? PO must have been a pretty big wheel to be able to afford something like this.
Anyone know how many were built? What are the specs? What speakers are in the console?
Are you using it in the console or with outboard speakers? How about some more pics of the TT and preamp? (I assume you have them, based on early pics).
I'd like more info on this amp/console too. Do you know the history? PO must have been a pretty big wheel to be able to afford something like this.
Anyone know how many were built? What are the specs? What speakers are in the console?
Are you using it in the console or with outboard speakers? How about some more pics of the TT and preamp? (I assume you have them, based on early pics).
jdsalinger
Super Member
Nevermind about the pics. I just checked out your flickr account. Wow, the wall of consoles is impressive!