Ok, so instead of resistors by the board, I should do this...
But it looks like I had the general idea right.
But it looks like I had the general idea right.
5150 Mod Thread
- Thread starter Wolfeman
- Start date
I guess it would also help if I said that I have a 5150 II, thanks! Also what did you do with the coupling caps, if you don't mind explaining it (sorry everyone I know pretty much nothing about tube amp design). Thanks so much!
TheWinterSnow
Den Mørke Natt
I haven't calculated the preamp stages, but I was wondering if off hand Wolfe you knew where specifically the fizz is coming from the amp AFTER a bias mod has been done. I know that its going to be part of the gain staging in the preamp, and I was hoping that I could do some mods to get the gain to be more musical. Just trying to get an idea as where to start. I am thinking about tweaking the miller capacitance and seeing how that works out, but thats going to take some pretty crazy calculations to make sure I am not fucking up the voltage/current gains of each stage as well as impedances.
Also, We discussed this before, I was wondering as to the specs of the power transformer to see if it can handle the current of modding the cathode heaters to DC to get a reduction of AC hum. I would also like to add a choke, but I am worried about the voltage reduction to the plates of all the tubes in doing so, particularly the power section as I would like to keep it at 500v, not sure the amp can be biased correctly if you drop it too much. Again, I could do the calculations, but I just wanted to see if you had a general idea to see if anything was a waste of time.
Also, We discussed this before, I was wondering as to the specs of the power transformer to see if it can handle the current of modding the cathode heaters to DC to get a reduction of AC hum. I would also like to add a choke, but I am worried about the voltage reduction to the plates of all the tubes in doing so, particularly the power section as I would like to keep it at 500v, not sure the amp can be biased correctly if you drop it too much. Again, I could do the calculations, but I just wanted to see if you had a general idea to see if anything was a waste of time.
PaoloJM
Member
The choke only effects the screen and preamp HT voltages.
The choke increases these voltages. The existing resistor is 470 ohms, the choke will have a DC resistance of around 125ohms depending on the model you go for and so drops less voltage. I'd recommend increasing the 100 ohm screen resistors, I personally go for 1kohm. Increasing the screen voltage will put more pressure on the valves and 100 ohm is far too low anyway.
You'll be able to bias up just fine.
Most of the preamp cathode heaters are already on DC.
For lead channel fizz reduction try a 500p cap across R86.
Hope that helps.
MarkG
Member
Too bad you have the combo, I love the light changes in the head
Tightens up the low end nicely.
If you don't like the range on the EQ, might be an easy mod to increase the range.
Tightens up the low end nicely.
If you don't like the range on the EQ, might be an easy mod to increase the range.
sprack
Thread Killer
On the topic of chokes; anyone have advice on placement? I've read that you want it non-planar with the bells on the OT. Is there a real mag-field issue or is it just voodoo? Asking because placing it in the middle of the chassis will require removing the main board, close to the OT and there's open space underneath.
sprack
Thread Killer
PaoloJM
Member
C58 is the stage coupling cap, it's not across R86. The 500p would go across R86.
Here's where I put the choke. The board does need to come out for this as you can see, but it's not a huge deal.
You can see where the new holes are drilled, where the cables comes through and the orientation of the choke.
Here's a quick camera phone clip of this amp.
Last edited by a moderator:
TheWinterSnow
Den Mørke Natt
From what I was reading on Aiken amps, a choke actually drops the voltage, obviously because the choke has a DC resistance (in series to the screen and preamp) from the windings, acting as a resistor, this maybe mild compared to the effective plate resistance, but I really don't want to see my screen voltage and preamp supplies dropping too much as a result of adding a choke.
From reading the "Radiotron Designer's Handbook" regarding Tetrodes and Pentodes, the Screen voltage is used to lesser the effect of grid-plate capacitance to enable more stability with higher frequencies or higher amplification. Adding a higher resistance on the screen should decrease the screen voltage therefore increase grid-plate capacitance, which at least to my knowledge is a bad thing as is lowers the amplification of the tube.
EDIT: I am having a hard time understanding why you would add a plate bypass capacitor on V5a when V5a and V5b are in parallel, you would still be getting the unattenuated signal of V5b. Also, placing the bypass cap there would not only effect the lead channel, but the rhythm and crunch as well.
PaoloJM
Member
The choke drops voltage compared to having a straight wire. They typically having a resistance of around 125 ohms depending on the model. It increases the voltages when compared to putting it in place instead of a 400 ohm resistor, which is what you're doing here in the 5150. It's basic Ohms Law.
The load (screens and preamp) remain the same, while the series resistance is being dropped. Therefore less voltage is dropped.
What schematic are you looking at?
V5a and V5b are in series driving the tone stack in both the 5150 and 5150II, so I'm not sure what you are talking about there. V5a is using a lot of local feedback, perhaps that's what's confusing you?
I'm looking at the 5150II here.
On the 5150II/6505 it will affect only the lead channel, on the 5150 it will both. On the 5150 you can't do much to the lead channel without affecting the other channel due to nature of the design.
Omega_Void
Your Eventual Destroyer
Too much screen current is bad, so 100R really is a bit small. PauloJM's recommendation of 1k won't reduce the output too much, but the screens will run cooler.
TheWinterSnow
Den Mørke Natt
I looked at it again and noticed where the choke would go on the supply circuit, but I cannot see a 400ohm resistor. The schematics are written pretty weird.
So decreasing that filter voltage will increase screen voltage and preamp voltage, however, I think I need to recheck those voltages because when I got my 5150 two of the screen grid resistors were bad and I had to replace them. When I was checking voltages my screen gird was equal to plate voltage which now studying electronics now I know is not a good thing. So that kind of scares me to place a choke without doing something about lowering the screen voltage. Definitely upgrading to 1k SGRs should remedy that. It will be a mod that I will constantly have to tweak.
Its a double edged sword, the voltage has to be lower than plate resistance but it can't be too low as it decreases stability. Its one of the areas in electronics thats more of an art than a science.
Rushtallica
New Metal Member
- Oct 17, 2010
- 8
- 0
- 1
Here's some other modding info posted at another forum:
http://www.rig-talk.com/forum/viewtopic.php?f=3&t=80183
And a thread with a clip of one (non-boosted) before and after a couple of the mods:
http://acapella.harmony-central.com/showthread.php?t=2690615
And the standard "working on amps can be dangerous; don't do it unless you know how to work on an amp without being electrocuted" warning applies here...
Added edit: I kind of got caught up in just reading about the mod info here and posted the links after just reading them, as well, but now realize this may be a bit OT to link in the same thread. If so, please let me know and I'll delete the post.
And thanks for putting up the info, Wolfeman. I don't currently have a 5150 anymore but plan to get another one soon to mod.
http://www.rig-talk.com/forum/viewtopic.php?f=3&t=80183
And a thread with a clip of one (non-boosted) before and after a couple of the mods:
http://acapella.harmony-central.com/showthread.php?t=2690615
And the standard "working on amps can be dangerous; don't do it unless you know how to work on an amp without being electrocuted" warning applies here...
Added edit: I kind of got caught up in just reading about the mod info here and posted the links after just reading them, as well, but now realize this may be a bit OT to link in the same thread. If so, please let me know and I'll delete the post.
And thanks for putting up the info, Wolfeman.
I don't currently have a 5150 anymore but plan to get another one soon to mod.
sprack
Thread Killer
TheWinterSnow
Den Mørke Natt
yeah they stick it near the OT on the schematics because the resistor is near the tubes rather than in the filter circuit on an entirely different board.
Here's some other modding info posted at another forum:
http://www.rig-talk.com/forum/viewtopic.php?f=3&t=80183
And a thread with a clip of one (non-boosted) before and after a couple of the mods:
http://acapella.harmony-central.com/showthread.php?t=2690615
And the standard "working on amps can be dangerous; don't do it unless you know how to work on an amp without being electrocuted" warning applies here...
Added edit: I kind of got caught up in just reading about the mod info here and posted the links after just reading them, as well, but now realize this may be a bit OT to link in the same thread. If so, please let me know and I'll delete the post.
And thanks for putting up the info, Wolfeman.
I want to note on the second link that the guy that modded his 5150 placed a 100pF cap between pins 7 and 8 on tube V5b. He was corrected as it should have been pins 6 and 7, which would increase the miller capacitance of the grid-plate, in effect, lowering the resonate frequency of the low pass that is created from said miller capacitance.
This mod would have a similar if not identical effect as adding a plate bypass cap on R86, the plate resistor of V5b.
sprack
Thread Killer
Whoever drew the 5150 schematic did so with the intent that no one could read it again. It's awful.
I hate reading schematics where the components aren't searchable, I've gotten spoiled.
I hate reading schematics where the components aren't searchable, I've gotten spoiled.
Omega_Void
Your Eventual Destroyer
Does kinda suggest they were getting a bit hot.
Yeah. You could always place the choke in addition to the resistor, though. Or go crazy and use a BJT/MOSFET regulator.
Undersized screen-stoppers aren't good for stability either.
Especially if they're metal film, since below about 470R they'll typically show an inductive peak at RF. 1k and up should show a purely capacitive rollof in RF impedance. Carbon comps (which I assume Peavey used) don't look inductive at any value, but they're noisy.
Anyway, you could go much higher than 1k before you'd be dropping too much voltage. The tonal change due to increased dynamic compression would be more likely to be a problem before then.
