How does voltage affect output transformer?

[hesamadman]

After months of tinkering back and forth on an el84 driven 30 watt I've came to a couple conclusions. This particular amp has a 30watt OT from a JTM30. I just recently found out that this OT specifications show 240v on the primary. My amp is giving it 346. First off.....this OT has amazing crunch tone but it has WAAAAY too much bass and I can't dial it out. I've tried everything. I've determined that it is coming from the OT since it's the last in the chain. Could adjusting the voltage going in help? Or is this OT just not going to work since it's not even meant for this tube type?

[jjasilli]

Info is not adding up.  JTM30 has 2X 5881 power tubes @ 240V is only about 15W output.  Are you saying Your amp is 30W output?  How many EL-84's are you using?  This affects plate-to-plate load impedance.  You need to post a schematic of your amp.

[VMS]

Did you take the 240V number from this hammond replacement transformer:

https://www.hammfg.com/files/parts/pdf/1750NA.pdf

I think the 240 is just the voltage they tested the transformer with.

[hesamadman]

Did you take the 240V number from this hammond replacement transformer:

https://www.hammfg.com/files/parts/pdf/1750NA.pdf

I think the 240 is just the voltage they tested the transformer with.

I was about to say that. Yes I got it from the spec sheet. So OT knowledge is something that I lack and I don't have a lot of literature on them to research so I'm going to take this opportunity to try and learn a little here.

plate-to-plate load impedance. 

Is there any chance you could explain this a little to me.


My amp is a custom preamp using the Hoffman AC 30 power amp. http://el34world.com/Hoffman/files/Hoffman_AC30.pdf

[jjasilli]

Plate-to-Plate Load Impedance is the specific primary impedance of the OT that the plates of the power tube(s) would like to see.  (It is NOT the internal plate impedance of the power tube(s), which is usually much higher, and varies with the circuit design).  The Plate-to-Plate Load Impedance is shown in the tube charts for typical operation.  Deriving these values on your own requires extensive mathematics.


The tube charts show the recommended Plate-to-Plate Load Impedance for typical B+ voltages.  For PP the value of 2 tubes is shown.  If you use four tubes, they are in parallel, so divide the tube chart's Plate-to-Plate Load Impedance in half.  Your OT's primary impedance should equal that, and not be more than 50% off either way.  Hope that helps.

[VMS]

First off.....this OT has amazing crunch tone but it has WAAAAY too much bass and I can't dial it out. I've tried everything.

What values did you use for C9, C10 and C11 on hoffman schematic?

[hesamadman]

Plate-to-Plate Load Impedance is the specific primary impedance of the OT that the plates of the power tube(s) would like to see.  Hope that helps.

This helps a ton. So according to tube data sheet for the el84 in PP at class A it shows a plat to plate load impedance of 10,000. Additional tube in parallel would be 5000. In technicality is an AC30 class AB? In which case the data sheet shows 8000 so in my application im looking at 4000. According to my spec sheet on the OT I am using it says 6000 to CT. Does this seem like im reading the spec sheet correct?


After looking at a the 30 watt OT specs for a quad el84 I see that its primary is 4000. So that is a big difference. It was a fun experiment trying the jtm30 OT but proven to be non satisfactory. I believe this is something that Sluckey tried to explain to me back in November when I was just getting parts for these amps but I didnt quite understand.

First off.....this OT has amazing crunch tone but it has WAAAAY too much bass and I can't dial it out. I've tried everything.

What values did you use for C9, C10 and C11 on hoffman schematic?

On all my CC I have .022 Ive dropped every value in the signal chain to try and reduce this bass because I really do like the tone of this OT but with the gain passed noon it starts to get very muddy.

[VMS]

On all my CC I have .022 Ive dropped every value in the signal chain to try and reduce this bass because I really do like the tone of this OT but with the gain passed noon it starts to get very muddy.

You can go even smaller, try 1nF or even 500pF cap on the C9.

[jjasilli]

hesamadman,  You have the right idea.  But Class of Operation is a matter of Bias; and proper Bias depends on Supply Voltage & the Current through the tube. 

At 346V, EL-84's cannot be biased to run in Class A.  That voltage is technically over-spec and off the charts.   That's OK , you're in Trainwreck territory.  You're probably in Class AB, so use that portion of the tube charts.  Looks like a PP pair should want about 8000 Ohms plate to plate load impedance; so a PP quad wants 4000 Ohms.  Your OT says 6000 which is within 50%.  Should be OK.

Also, I agree with VMS' approach.  I doubt your tone issue is due to the OT.  It's probably caused elsewhere.  Since you say you have a "custom" preamp, that's where I too would focus.
   

[hesamadman]

You can go even smaller, try 1nF or even 500pF cap on the C9.

EXTREMELY good call. I swapped the very first CC from .022 to 500p like in an ac30 pre and I chanced C9 to a 500p. It took all the un-wanted bass out. This thing sounds like a gnarly high gain amp. Its rad for a 30 watter. Cant believe how much that cleaned up the distortion. I may go up a few notches from the 500p though as I may have taken a little too much out but that really helped.

[scrimpus]

I use a .001 on most of my Voxish builds.

[hesamadman]

I use a .001 on most of my Voxish builds.

Do you use them on all CC locations? Right now I have a 500pf on the first and a 500pf before the PI. Im wondering if I should make the .022 that are left in the chain .001


This particular amp is a somewhat higher gain amp with an AC30 power amp. Its setup like a 2204 preamp using that tone stack but using a PPIMV. The amount of preamp tubes are the same but values are non related to a 2204. I just added a 1uf cathode bypass cap on the second half of first triode. It created a little bass....which is what i am trying to eliminate.... but it added a sweet gain emphasis. As this amp is geared toward a thrash/metal tone i like the gain i get....but having trouble getting happy medium of gain and low end.

[VMS]

I think you should first change those 500pF capacitors to 1nf capasitor because you reported that you liked the result but it went little too far so you tweak those components.

1uF bypass cap gave you good distortion but little too much bass so you can change that to 680nf, 470nF or 330nF so that you have the distortion but not so much bass.

It is always good to change one component at a time and listen to the result, that way you know how that component value affects the sound.