Various amp questions

[tubenit]

My VVR on the Tweed Overdrive Special started sounding fizzy/gritty for some reason so I removed it.  When it was installed initially, it sounded excellent down to about 2.5-3. Below 2.5-3, it sounded fizzy. Well it got to where it sounded fizzy at almost anything below 7 even though it worked & still reduced the volume just fine.

I installed a LarMar PPIMV instead & it sounds really good down to about 5. Below 5, the tone changes too much for my taste but I am going to leave it in. I don't hear any tone change between 7-10. It gets progressively thinner sounding closer to 5 & below 5, it sound somewhat anemic to my ears.

Q1)  Is there going to be a problem with the 250R/5w resistor that I've circled. It gets pretty hot. You can touch
       it but definitely can't leave your finger on it after 20 minutes of playing. It was left in there because the VVR
       required adding more filtering.

Q2)  I noticed the pot's resistance on the dual gang 250kl does NOT match. For example one pot is 252k and the other
       is 231k. Has anyone run into this before?  I've found this type of discrepency on 3 different dual gang pots.
       What I did was measure the differences between 5-10 and installed a resistor to ground to help compensate.

       Does 252k & 231k to ground make any real difference in tone?

With respect, Tubenit

[sluckey]

I'd measure the voltage across the 250Ω resistor then calculate power. P = E²/R

This gives the actual power dissipated in that resistor. For a safty margin, choose a resistor to handle at least double the actual power.

I'd use the dual pot as is. The circuit won't mind. Can't say about tone, but I'd think it would make very little to no difference.

[tubenit]

Sluckey,

THANK YOU! I always appreciate your responses.

I'd measure the voltage across the 250Ω resistor then calculate power. P = E2/R

I have never had any training in electronics.      Everything I've learned has been thru the forum, reading and experimenting.

I can do math reasonably well, but I confess that I am not sure what numbers to plug into the "E2" or the "R"?  I am assuming the R= 250 ohms?  Is E2 = 23 volts?       (371-348 = 23)    Does "P" equal wattage?

Can someone help me out with the formula please?

That 250R/5w resistor drops voltage from 371v to 348v.


The "adjusted" resistance on the dual gang 250k pot seems to work out just fine. It sounds great.

With respect, Tubenit

[sluckey]

E is the voltage across the resistor, 23 volts in this case. E2 is really E², or E squared.
R is the resistor value.
P is power in watts.

P = E²/R = 23x23/250 = 2.1 watts.
Double that for safety = 4.2 watts.
I'd say your 5 watter is fine.

[tubenit]

Sluckey,

I sure appreciate your help with this!  THANK you!  Sounds like I was headed the right direction, but I wasn't sure?

That's all I needed to put the amp back together.

With respect, Tubenit

[TIMBO]

Hey Tubenit, I think your concepts are great and have learn't alot since joining this forum.The TOS has evolved nicely from your original idea and the changes has inspired me to cut out the CRAP and increase the tone  So a REBUILD is in order, the schem your have just posted is what i will be aiming for as the rebuild and the amp as it is will transform nicely.

The topic you are discussing (250r/5w) i have in place a 3H choke as per my original build. Are there any pros or con to having a choke v resistor. What does it relate to TONE or amp PERFORMANCE. (SAG) Looking forward to cutting out the CRAP.

"CRAP" The stuff you don't see on the schem that i add because i thought it would enhance the "TONE" 

Thanks

[tubenit]

Timbo,

I posted the new schematic and layout in the SCH library.  

I am getting a very clear transparent overdrive with nice harmonics, great touch sensitivity, good sustain and some blooming (where the notes swell and fill out).

I originally started with a 150uf on the power tube cathode.  I now have a 22uf.  I was stunned at the difference in tone & the lower 22uf value allow quite a bit more blooming and harmonics in my amp. I just kept hooking up different cathode caps until I found the value that was best to my ears. The 22uf is much smoother tone, IMO.

I also prefer the V2 triode to be the CF after the 5879 instead of fat triode. With the CF after the 5879, I did not need my FX-reverb unit anymore as the CF sounded every bit as good with the Boss digital delay in.

The original TOS sounded very good to me. However, I think where I have it now is quite a bit better and more transparent sounding. It really allows the guitar to be more expressive without sacrificing overdrive.  

I think the choke is fine. The TOS that I sold had a choke in it.

Just my two cents worth.    with respect, Tubenit

[HotBluePlates]

Because the whole amp's current flows through that 250 ohm resistor, a choke in its place will yield less sag while offering even better filtering.

[PRR]

> You can touch it but definitely can't leave your finger on it after 20 minutes

That's not hot. It is a block of ceramic. If it glows dull-red in the dark, that's pretty hot. Electronic resistors should not be dull-red because the markings burn off (and the expansion may stress the carbon guts). Also, and mainly, so the resistor does not set wires on fire.

A fully-used power resistor should sear your skin and keep a little of it, like a steak thrown on a very-hot grill and plucked right off. OTOH, it should not ignite a kleenex touching it.

You are fine there.

Your pot question: surely these are used as 3-terminal potentiometers, not variable resistors. So what matters is that the _ratio_ is similar between sections. Wire the ends to 9V. Set the wiper to the middle (best you can). If both say about 4.4V, it's matched good enough.

> VVR ... sounded fizzy

What is the load impedance? If less than 8K, it isn't well loaded for the current you have. Since the V/I ratio of the tube changes with VVR varying B+ down, it sure could sound anemic down in the 100V range.

[tubenit]

PRR,

THANKS for the response and comments!  They are appreciated. That gives me a good reference point to know what is within normal limits.

With respect, Tubenit