High voltage Hiwattish build progress

[lord preset]

I've been ruminating for a while about building a Hiwatt variant using a PT/OT set from a 1980's KMD that was a close clone of a 100 watt Musicman. The transformers are designed for 700 plates and 350 volt screens. Things are starting to come together. The preamp follows the Trinity Triwatt design and the voltage doubling PS comes from schematics and help I got from TAG. I am going with 2-6L6GCs as this it what the KMD ran and I don't have specs on the PT, so I don't know for a fact if the filament tap has the current to run other tubes like KT88s or EL34s. I won't get 100 watts without the SS driver but I'm thinking 75 might be reasonable.

A build would not be mine without at least one good #^# up. In this case I cut the IEC hole in the wrong spot. The only way to avoid trashing the chassis was to flip the layout and have the front panel run right to left in the Marshall style rather than left to right. I resolved that I would pretend I intended to do this all along. When I went to wire up the board I realized that when you flip the board layout the tube sockets do not flip, meaning that wires that ordinarily would not cross now do.

The last major wiring job is running the heater wires. Then fire it up and watch the city go dark. One questions I do have at this point is if it matters whether NFB is run off the 8ohm OT tap vs the 16 ohm tap as called for in Hiwatt and Triwatt schematics. The OT I am using only has 4 and 8 ohm taps.

[lord preset]

Haaalp!!

I am seriously stumped here. Hopefully there is some idiocy on my part rather than a more serious problem. I finished wiring up the amp and went to do a tubes-out light bulb limiter test using a 60 watt bulb. Flipped the switch and there is a steady medium bright orange glow. I disconnected the heater wires and still a glow. To make a longish story short I have now disconnected all the PT secondaries from the circuit and the glow remains.

Some background. Prior to the light bulb test confirmed that I was getting expected voltages from the PT secondaries. I then installed and and tested the PS board, rectifier board and switching power supply. All the unloaded voltages look good and no blown fuses. Nevertheless, now with nothing connected to the PT secondaries the #$#@ limiter bulb continues to glow happily. Presumably if I had used the light bulb limiter earlier in the build process I would have seen this earlier but here I am.

Am I missing something really obvious or could the PT be shorting to ground in some way? I can't see why the PT would be drawing significant current with no load.

[lord preset]

Haaalp!!

I am seriously stumped here. Hopefully there is some idiocy on my part rather than a more serious problem. I finished wiring up the amp and went to do a tubes-out light bulb limiter test using a 60 watt bulb. Flipped the switch and there is a steady medium bright orange glow. I disconnected the heater wires and still a glow. To make a longish story short I have now disconnected all the PT secondaries from the circuit and the glow remains.

Some background. Prior to the light bulb test I confirmed that I was getting expected voltages from the PT secondaries. I then installed and and tested the PS board, rectifier board and switching power supply. All the unloaded voltages look good and no blown fuses. Nevertheless, now with nothing connected to the PT secondaries the #$#@ limiter bulb continues to glow happily. Presumably if I had used the light bulb limiter earlier in the build process I would have seen this earlier but here I am.

Am I missing something really obvious or could the PT be shorting to ground in some way? I can't see why the PT would be drawing significant current with no load.

[eddie25]

I would put like a 1 amp fuse in line with the PT like normal and try it with no secondaries connected still. If that survived I would probably move on.

[lord preset]

I would put like a 1 amp fuse in line with the PT like normal and try it with no secondaries connected still. If that survived I would probably move on.

I can try that. But is it within the range of normal for an unloaded PT to draw enough current to light up the limiter?

Maybe I should try licking it.

[eddie25]

Well, I think either the transformer is fine and you are seeing a normal amount of glow, or you have a short in which case it would be glowing pretty nicely. Since you had proper voltages and nothing weird has happened since then, I'm gonna bet its fine and you can try with just a fuse.

[lord preset]

I would put like a 1 amp fuse in line with the PT like normal and try it with no secondaries connected still. If that survived I would probably move on.

I tried a 1 amp fuse and it popped. With a 2 amp fuse it does not pop. With the PT isolated from the rest of the circuit I couldn't see any potential shorts but to be absolutely sure I removed the PT from the amp and and connected it to AC power with an inline fuse. Same behavior. 1 amp fuse pops, 2 amp does not.

The PT hums a fair bit when energized. I am wondering whether it could simply drawing a fair amount of power in thermal and mechanical energy. Or is the PT shorting internally? FWIW, The mains fuse on the donor amp was 5 amps.

So the question for me is should I use a PT that pops 1 amp fuses unloaded, or am I scroooood.

[Leo_Gnardo]

or am I scroooood.

No PT with no load on the secondaries should pop a 1 amp fuse. Buzzes, yes, expected with a damaged PT. And if you insist on leaving it on, may overheat & start smoking & stinking up the room. Live & learn I guess

[lord preset]

or am I scroooood.

No PT with no load on the secondaries should pop a 1 amp fuse. Buzzes, yes, expected with a damaged PT. And if you insist on leaving it on, may overheat & start smoking & stinking up the room. Live & learn I guess

I don't insist. I'm just bummed. I built this amp around that PT, with voltage doubling PS and all. If I had known I would have to buy new iron it would have done things differently. But I assumed that since the donor amp seemed to be OK when I got it the PT should be good. Possibly it was in the early stages of failing.

I guess I can go get a MusicMan PT from Mojo...



Grrrr..

[lord preset]

It's been a while since I posted on this but it hasn't been dropped. After determining that the PT I was try to use was having "issues" I set about sourcing a replacement Music-Man type PT. Mojotone has long been the source for these but I learned that their supply has dried up, at least for the model I needed. Was under the impression that Heyboer was making MM iron for Mojotone but apparently that may only be true for some OT models. Mojotone said they have been selling NOS stock they acquired years ago and are running out.

So I went to Heyboer - they didn't have anything in stock, and since they have to make one up for me I decided to change some specs - higher 6/3v current, 5v tap etc to make things more to my liking. But since anything worth doing is worth hosing up I managed to mis-specify the B+ spec. Rather than writing "270-50-0" I wrote 270-50-0-270". When the PT arrived it was somewhat bigger than I was expecting. Hmmm, why would that be? No matter. So in the intervening time waiting for the new iron, I worked on the different build and my tenuous grasp of the voltage doubling PS design on this build got fuzzier. So I confidently hooked the sucker up, fired up the mains and checked heater bias and switching voltages - all good. Then I hit the standby an the light bulb limiter went bright and stayed that way.

I will stop here to thank whoever thought of the light bulb limiter. Thankyouthankyouthankyou.

I took me two days to finally realize what I had done wrong. Fortunately nothing was destroyed due the light bulb, but I am left with deciding what to do. Two ideas come to mind re. reworking the PS scheme :

1. Use a full wave bridge rectifier with the center tap disconnected. From what I can tell this should get me ~ 756v unloaded, maybe ~650v loaded which is about right for B+. Not sure how I would get screen/preamp voltages down to ~400v though. Zeners? A VVR type circuit? Duct tape?

2. A more boneheaded idea that came to mind was to not use one of the 270v taps and instead use one 270v side with the CT as 0v and connect the PS as-is in voltage doubler configuration. It seems to me that it would work, but what do I know? But if it did work would I be limited to 1/2 the 400ma HT current rating?

[Phil_S]

1. Use a full wave bridge rectifier with the center tap disconnected. From what I can tell this should get me ~ 756v unloaded, maybe ~650v loaded which is about right for B+. Not sure how I would get screen/preamp voltages down to ~400v though. Zeners? A VVR type circuit? Duct tape?

2. A more boneheaded idea that came to mind was to not use one of the 270v taps and instead use one 270v side with the CT as 0v and connect the PS as-is in voltage doubler configuration. It seems to me that it would work, but what do I know? But if it did work would I be limited to 1/2 the 400ma HT current rating?

You didn't tell us what you did wrong. It is good form to share information so others will learn from your experience. That is the nature of the forum. You may get ribbed for it, but I don't imagine anyone will "get on you" about it. It is your decision to share or not.

On #1. You can probably drop the B+ for the screens with a choke and preamps using a large value resistor. I'm not sure how you figure the drop to 650V unless you told them that's what you needed and they wound it to do that. I usually figure 4-6% drop under load, which would put it into the low 700's.

On #2, you should check with Heyboer. I believe the answer lies in how they wound it. Since you paid for a custom wind, I'm sure they will share that with you willingly.

[lord preset]

You didn't tell us what you did wrong. It is good form to share information so others will learn from your experience. That is the nature of the forum. You may get ribbed for it, but I don't imagine anyone will "get on you" about it. It is your decision to share or not.

On #1. You can probably drop the B+ for the screens with a choke and preamps using a large value resistor. I'm not sure how you figure the drop to 650V unless you told them that's what you needed and they wound it to do that. I usually figure 4-6% drop under load, which would put it into the low 700's.

On #2, you should check with Heyboer. I believe the answer lies in how they wound it. Since you paid for a custom wind, I'm sure they will share that with you willingly.

I thought my mistake was self evident but hey I have no pride:
1. The big thing I did wrong was mis-specify the transformer.
2. Then I didn't realize it.
3. Then I hooked the two 270 HT wires up to the existing voltage doubling PS designed for 0-270 AND grounded the Center Tap.
4. Then the light bulb limiter saved my sorry ass.

Actually I think may be a good thing that I grounded the CT. With it grounded I basically had a short. If I hadn't I can only assume I would have gotten some hideously high B+ - like ~1500v - at least until something when kaboom.

As for #1 I was basing the ~650 on Lord Valve's discussion of full wave bridges where he talked about a reduction of 10-15% under load. Either way, low 700s would be fine. FWIW I will be running KT88s in this.

On #2, that's an excellent suggestion. I suppose Heyboer would know

[ToneMerc]

It's been a while since I posted on this but it hasn't been dropped. After determining that the PT I was try to use was having "issues" I set about sourcing a replacement Music-Man type PT. Mojotone has long been the source for these but I learned that their supply has dried up, at least for the model I needed. Was under the impression that Heyboer was making MM iron for Mojotone but apparently that may only be true for some OT models. Mojotone said they have been selling NOS stock they acquired years ago and are running out.

So I went to Heyboer - they didn't have anything in stock, and since they have to make one up for me I decided to change some specs - higher 6/3v current, 5v tap etc to make things more to my liking. But since anything worth doing is worth hosing up I managed to mis-specify the B+ spec. Rather than writing "270-50-0" I wrote 270-50-0-270". When the PT arrived it was somewhat bigger than I was expecting. Hmmm, why would that be? No matter. So in the intervening time waiting for the new iron, I worked on the different build and my tenuous grasp of the voltage doubling PS design on this build got fuzzier. So I confidently hooked the sucker up, fired up the mains and checked heater bias and switching voltages - all good. Then I hit the standby an the light bulb limiter went bright and stayed that way.

I will stop here to thank whoever thought of the light bulb limiter. Thankyouthankyouthankyou.

I took me two days to finally realize what I had done wrong. Fortunately nothing was destroyed due the light bulb, but I am left with deciding what to do. Two ideas come to mind re. reworking the PS scheme :

1. Use a full wave bridge rectifier with the center tap disconnected. From what I can tell this should get me ~ 756v unloaded, maybe ~650v loaded which is about right for B+. Not sure how I would get screen/preamp voltages down to ~400v though. Zeners? A VVR type circuit? Duct tape?

2. A more boneheaded idea that came to mind was to not use one of the 270v taps and instead use one 270v side with the CT as 0v and connect the PS as-is in voltage doubler configuration. It seems to me that it would work, but what do I know? But if it did work would I be limited to 1/2 the 400ma HT current rating?

I wish you had reached out to me before you ordered it, I could told you exactly what you needed and how to design it. What you have there is totally wrong for the application.

Contrary to what Mojo told you, Heyboer does wind the MM replacements. The
first HV build I did 4 yrs ago I used a MM replacement from Mojo which was wound by Heyboer. I could have just given you the Heyboer part number and you could had them to clone it.

The bias winding should be separate from the HT winding, I typically order only a 32-35V tap, then just like a Marshall ground one side of the winding and config it just like a normal Fender fixed bias supply.

The HT current needs to be twice as much as you think, the smallest MM PT's were just shy of 600mA, the bigger ones in the range of .900-1A.


Wire the supply just like the diagrams i posted and you are golden.


TM

[lord preset]

I wish you had reached out to me before you ordered it, I could told you exactly what you needed and how to design it. What you have there is totally wrong for the application.

Contrary to what Mojo told you, Heyboer does wind the MM replacements. The
first HV build I did 4 yrs ago I used a MM replacement from Mojo which was wound by Heyboer. I could have just given you the Heyboer part number and you could had them to clone it.

The bias winding should be separate from the HT winding, I typically order only a 32-35V tap, then just like a Marshall ground one side of the winding and config it just like a normal Fender fixed bias supply.

The HT current needs to be twice as much as you think, the smallest MM PT's were just shy of 600mA, the bigger ones in the range of .900-1A.


Wire the supply just like the diagrams i posted and you are golden.


TM

Well there is no mistake like an expensive one.... Ordering a CT design was really just a typo on my part. In my head I was thinking 270-50-0. As for the HT current that's ignorance on my part - I couldn't find any specs on it and didn't ask here (or Heyboer for that matter). But the reason I didn't just ask for a MM replacement is that I was concerned that it might not have enough filament current for KT88s and 4 preamp tubes. I was thinking of the MM RD65/100 PT not the HD130 but I suppose if I got the HD130 version that wouldn't be a concern.


Anyway I would appreciate your Heyboer part number if you don't mind. Also, I am curious why the bias tap should be separate in this case, since it often isn't?

[ToneMerc]

I wish you had reached out to me before you ordered it, I could told you exactly what you needed and how to design it. What you have there is totally wrong for the application.

Contrary to what Mojo told you, Heyboer does wind the MM replacements. The
first HV build I did 4 yrs ago I used a MM replacement from Mojo which was wound by Heyboer. I could have just given you the Heyboer part number and you could had them to clone it.

The bias winding should be separate from the HT winding, I typically order only a 32-35V tap, then just like a Marshall ground one side of the winding and config it just like a normal Fender fixed bias supply.

The HT current needs to be twice as much as you think, the smallest MM PT's were just shy of 600mA, the bigger ones in the range of .900-1A.


Wire the supply just like the diagrams i posted and you are golden.


TM

Well there is no mistake like an expensive one.... Ordering a CT design was really just a typo on my part. In my head I was thinking 270-50-0. As for the HT current that's ignorance on my part - I couldn't find any specs on it and didn't ask here (or Heyboer for that matter). But the reason I didn't just ask for a MM replacement is that I was concerned that it might not have enough filament current for KT88s and 4 preamp tubes. I was thinking of the MM RD65/100 PT not the HD130 but I suppose if I got the HD130 version that wouldn't be a concern.


Anyway I would appreciate your Heyboer part number if you don't mind. Also, I am curious why the bias tap should be separate in this case, since it often isn't?

All replacement MM PT will run warm with just four EL34's as its only around 5A, smaller ones are slightly less and I even that Hd130 replacement would run very hot with four Kt88's,

If you are truly set on running KT88's you are going to need a much bigger PT. What is the size of your PT cutout? There's only so much wire you can physically wind on a frame. The lamination stack can be made taller but there will be a point where a endbell will not fot over the core. Keep in mind are talking over 1A of HT current and at least 8A of filament current plus bias and another 5V winding, this takes up room.

Dedicated bias winding will have better stability, being totally independant of any draw from the HT winding.

Here's the next question, what about at least a 150-175W output transformer which to matches the KT88 load in this configuration?


Here's the general guide of how you want to order a basic quad EL34 version that fits in a Twin Reverb or JTM factory cutout. Unlike the factory MM version with the underrated filament and CT bias winding, this runs cool.

120V primary

268-0 @ 1A

35-0 @ 300mA

6.3V @ 7A

TM