KT66 Rocket Build

[Blind Lemon]

Here are RJs voltages, if you built it like his you should be close.

BL

https://tubeamparchive.com/download/file.php?id=7549

[RJ Guitars]

Any comments about the screens being 7 volts higher than the plates? Should I try to drop the screen voltage before playing the amp again, or is it not that big of a deal? ...

TD, That is something new to me... which might just mean that I don't know much.

If indeed your B+ nodes are all in the right place, then something has to be dropping that plate voltage. Something has to be resistance?

When I went to compare voltages to my work I realized that I don't have any screen voltage measurements...

I did try to measure the DC resistance on the same model transformer... while I can't confidently tell you if the DC resistance is the final piece of the equation, the results are interesting.

My meter acts funky when measuring the DC resistance of large inductors and on some of these I had to measure from the center tap to each side.... what value did BL mention? How about ~ 95 ohms per side.

EDCOR = ~190 ohms
STANCOR = ~123 ohms
HEYBOER ToneSlut = ~71 Ohms
HEYBOER 8093-A (Dynaco) = ~188 ohms
DYNACO 470 = ~64 Ohms

I have put out a message to the Boozhound labs AudioNerdz asking about this... I will post anything I learn.

rj

[Blind Lemon]

RJs running 1K screens and TD is running 220R........

[tweedeluxe]

If I'm right you should see about 100 ohms to the CT per side. (5K)

BL

BL, can you explain this in more detail? I've triple checked that I have B+1 and B+2 wired properly. Does it appear incorrect in the photos? B+1 is wired to one pole of the standby switch - the choke and OT center tap are wired to the other pole. The other lead from the choke is soldered to the screen grid Rs and the second filter node.

I tried dropping the screen voltage with a "shared" resistor before the 220ohm screen grid Rs - I went up to 4k7 (which should be equivalent to double that, right?) and still the voltage wouldn't drop below the plate voltage (which stayed the same).

What am I missing?

BTW - I cranked the amp for a good half hour again last night (still with just 220R on each screen), and the screens do not appear to be glowing at all.

Thanks for all the help guys, I really appreciate it. Still trying to wrap my head around all this. Hopefully I can get it sorted out.

[tweedeluxe]

EDCOR = ~190 ohms

rj

Thanks, RJ - didn't measure that last night. The DC resistance of the choke is 416ohms - so I don't understand why the OT is dropping 10 volts and the choke is only dropping 1-2.

[RJ Guitars]

If I'm right you should see about 100 ohms to the CT per side. (5K)

BL

BL, can you explain this in more detail? I've triple checked that I have B+1 and B+2 wired properly. Does it appear incorrect in the photos? B+1 is wired to one pole of the standby switch - the choke and OT center tap are wired to the other pole. The other lead from the choke is soldered to the screen grid Rs and the second filter node.

I tried dropping the screen voltage with a "shared" resistor before the 220ohm screen grid Rs - I went up to 4k7 (which should be equivalent to double that, right?) and still the voltage wouldn't drop below the plate voltage (which stayed the same).

What am I missing?

Any comments about the screens being 7 volts higher than the plates? Should I try to drop the screen voltage before playing the amp again, or is it not that big of a deal? ...

...I have put out a message to the Boozhound Labs AudioNerdz asking about this... I will post anything I learn.

rj

Tweedeluxe, I don't think you are missing anything, but rather you have discovered something quite interesting. It wasn't until I went to share my voltages with you that I found that I had not written down any screen voltages, which means I never paid any attention to them. I'll be doing that today and see if I notice anything similar.

My plate voltages did vary dependent upon which tube I had in there so the KT-66 might indeed give a different variance than a 6L6GC... great stuff for us to sort out, but I understand how it can concern you when you believe you've done everything right. It's likely that you have done everything right judging from your description of the tones your getting.

Below is the response from Jason over at Boozhound labs...

rj

I think this is actually kinda common in guitar amps. I'm absolutely
not sure about it at all, but I have seen a lot of schematics where
there isn't even a choke between the OT and the second grid.

I'm guessing it will be just fine. The grid resistors will keep the
grids from drawing too much current when the amp is heavily
distorting.

Jason

[tweedeluxe]

RJ - thank you so much - I just breathed a huge sigh of relief. Great to hear that from someone who knows what they're talking about (ie: not me).

Looking forward to hearing what your voltages are.

[tweedeluxe]

BTW, out of the dozen or so preamp tubes I have (a few NOS but mostly NP), here is my fav tube combo:

V1 - GE 12AX7
V2 - JJECC803S (this tube seems to have a lot of gain - way too much for V1, but perfect for V2 (cathode follower).
V3 - Tung Sol RI 12AX7.

Just for kicks . . . I also auditioned the 3 Tungsram CV492s that I'm saving for my upcoming EL84 rocket build . . . WOW!

I was really amazed at how sensitive the amp is to tube selection - I'm not used to that.

[RJ Guitars]

Tweed - The key to success is always going to be having the right resources. My mind is not always the right resource for things that matter... but the Audionerdz, these guys are great! These guys are all vacuum tube HiFi building experimental geniuses... plus really nice guys! They have been my mentors and deep knowledge resource for years now (BTW - they are all younger than me).

Here is another response from a guy named Matt - another mega smart dude that loves to experiment...

Bob (rj in this world),
Screens are normally higher on a P-P output transformer because they
are closer to the B+ tap, so less drop due to ohms law. The plate tap,
which is the full coil will be lower. I'm not sure if this helps.
Matt

[Markusv]

Hi

Back after a few days

Tweeddeluxe's experience is also mine
I have to explain my setup first though

Rectifier 5U4
B1 Node-- OT Center tap
1K/20 watt resistor between nodes 1 and 2 (instead of choke, as in l/pool)
B2 Node -- Screens
Etc......

My voltages differ by roughly 2-3 volts, the screens being lower than the plates

Previously, I only had 500ohm between Nodes 1 and 2 and the screens were same or higher.
(So , between the B1 node and the plates I had a voltage drop that was slightly greater than there was between B1 and B2)

Did 2 things:
Increased the 500R resistor between B1 and B2 to 1K
Increased my screen resistors to 1K

Now the screens are lower by a few volts

BTW I run Shuguang KT 66's and they are incredible!
There is a girth there that I have not detected with the 6L6's before, plus the Boominess of the bass is gone
The mids and treble are smooth and I think these are very well- balanced tubes

Thanks for the suggestion!

Will report back with a full voltage breakdown later

Markus V

[Markusv]

Screens are normally higher on a P-P output transformer because they
are closer to the B+ tap, so less drop due to ohms law. The plate tap,
which is the full coil will be lower. I'm not sure if this helps.
Matt

[/color][/i]

I assume this is because they run the OT ultralinear?

Markus V

[RJ Guitars]

Any comments about the screens being 7 volts higher than the plates? Should I try to drop the screen voltage before playing the amp again, or is it not that big of a deal? ...

TD, That is something new to me... which might just mean that I don't know much.

If indeed your B+ nodes are all in the right place, then something has to be dropping that plate voltage. Something has to be resistance? ...blah, blah, blah
rj

Well, Ohms law says that if you have a voltage drop, it can only be one of two things... I chose the wrong one. It's not that the resistance went up.

The B+#1 node is feeding two very current hungry KT66 tube plates. Meanwhile the B+#2 node is feeding a bunch of lightweights. Since both nodes start with a pretty similar voltage and the resistance is fixed... you only get one option. B+ at the plates is going to drop.

O.K. mystery solved... but wait. How can the amp work if the plate voltage is lower than the screen voltage?

from my friend Matt of the AudioNerdz group - this is cool!

Bob,
I'm about off overseas and wont be back for a week.
The only current pulling through the choke is 5 12AX7 sections which
are probably no more then 1.5ma each (I don't have the time to
calculate from plate curves) so 7.5ma plus maybe 3ma going through
those KT66 screens, 10.5ma total. I would not expect the voltage drop
across the Hammond 158L to be much at all. One KT66 plate is probably
pulling at least 40ma and I would expect the drop across half the
output winding at that current to be larger then the drop across the
Hammond. So I would expect the plate voltage at the tube pin to be
more then 10% lower then the voltage on the screen.

As drawn this is
not really pentode operation, the screen and plate of the KT66 are
really triode connected by the 1K resistor plus the DCR of the Hammond
and output coil. There is no AC ground on the screen like there should
be for pentode operation so the screen and plate track B+ together.
Matt

[RJ Guitars]

Screens are normally higher on a P-P output transformer because they
are closer to the B+ tap, so less drop due to ohms law. The plate tap,
which is the full coil will be lower. I'm not sure if this helps.
Matt

[/color][/i]

I assume this is because they run the OT ultralinear?

Markus V

Correct, Matt assumed Ultra Linear but I sent him the schematic and he sorted it out for us real quick...

rj

[Blind Lemon]

So basically the KT66s are running in triode mode.?

Bob

[tweedeluxe]

I don't know for sure, but it appears so. Is there really no AC ground on the screens though? Seems like there would be from the filter cap - but I'm probably wrong.