Fibre board just say NO!!!

[WhopperPlate]

Noise issues with SVTs seems to be common with mismatched tubes, something not hard to do with a sextet of 6550s. I've had several SVTs, both new and old, on my bench for repair and they are more than capable of a low noise floor.

Even trickier with out of production 6146b tubes! Hum balance and current matching per side becomes imperative .

[Mark]

You know this comes up right as I was considering this factor with Bakelite components .

Bakelite resistance can increase with humidity and heat. One study I read showed a decrease from 1000m to 60m at one environmental extreme.

This crossed my mind that this isn’t ideal for components that are often typically mounted on Bakelite terminal strips , like high voltage power supplies .

I am building an SVT style bass amplifier right now with the 700vdc power supply , and I have opted to build g10 turret strips where ever possible for close mounting components at the socket pins . With so many environmental variables , why bother building something that will operate even more inconsistently? It gets HOT and it SHAKES. Inconsistency becomes apparent when you are torturing your gear with regular usage getting thrown around from gig to gig. Might not matter as much for guitar Joe playing crunchy blues rock out of his 50 watt combo, but to get a clean powerful bass response these issues will undoubtedly be more noticeable to a discerning ear .

To me it seems willfully negligent to utilize sub par board material, when we have stuff like g10 . I just use the old boards for making drilling templates for the G10 lol

I would have a good look at the SVT circuit before building it. It has some abnormalities in it that I don’t like.
I don’t like the preamp out power amp in section. They boost the signal up through the preamp only to attenuate it down to 0.6v, the power amp section then has to amplify the signal back up again. I have found there is hum in the SVT that really shouldn’t be there.

Thanks for the care and fair warning ! I am not cloning the thing ; I am taking more than a few liberties . Client has a few blue lines and I am servicing those as well. there is a good amount of tweaks to the SVT that can remedy hum problems.

I would like to hear about the tweaks you made?
The tweaks I made was replacing the bias pots with ten turn pots. That really helped in setting up the bias. I twisted the heater wires where I could but that didn’t make any difference.

[WhopperPlate]

I would have a good look at the SVT circuit before building it. It has some abnormalities in it that I don’t like.
I don’t like the preamp out power amp in section. They boost the signal up through the preamp only to attenuate it down to 0.6v, the power amp section then has to amplify the signal back up again. I have found there is hum in the SVT that really shouldn’t be there.

Thanks for the care and fair warning ! I am not cloning the thing ; I am taking more than a few liberties . Client has a few blue lines and I am servicing those as well. there is a good amount of tweaks to the SVT that can remedy hum problems.

I would like to hear about the tweaks you made?
The tweaks I made was replacing the bias pots with ten turn pots. That really helped in setting up the bias. I twisted the heater wires where I could but that didn’t make any difference.

Well the grounding can sometimes go whacky with the send return jacks ground return point .this can be moved to a bolt near the can caps for a sturdier connection . Grounding patterns are apparently inconsistent and a source for headaches in these amps over the years

The preamp shielded send cable can be isolated from the rest of the wiring bundle , but that may not be enough as they still enter and exit through the same chassis holes. Complete isolation would be a more sensible approach logically speaking

Lyle from Psionic audio has a some good repair videos on the amps, where he mentions all this , and he was adviced by another tech to remedy the persistent hum he was getting in one amp by changing the .1uf CF send cap to a 1uf to lower the output impedance on the shielded run and lower interference susceptibility . It worked well for that amp.

One of those blue line SVt that my client has still has entirely original caps and 6146b tubes . A recent Craigslist purchase. Thing puts out and purs with how clean , soothing and fat the bass gets . Looks like only the output plate resistors and screen resistors have ever been touched. Hummed real bad before the client turned the hum balance all the way one way lol . Now it’s even quieter than their workhorse SVt lol that’s been serviced plenty before (albeit not by me lol). I am going to give that thing a careful servicing with all that in mind , but moral of story : these things can run surprisingly quiet in spite of expectations and circumstances .

[pjd3]

Its funny,

The local amp tech here in the north Boston area, who actually has some pretty big guitar names personally drop their amps off to him swears that part the reason old Fenders have a sound that is really special, is due to the fiberboards introducing all of these electrical characteristics that effect the sound, as if there are a handful of 2-3 Meg ohm resistors sprinkled about the board randomly connecting eyelets. I sure can't prove it or Un-prove it but, the guy has certainly seen the inside of far more vintage and modern amps than most people will ever see in 50 life times. And he plays guitar in a Pink Floyd tribute band.

Phil D

[martin manning]

There's no denying that there are lots of components in an amp that don't appear in the schematic.

[TUBEDUDE]

Now the internet will be on a quest to find fiberboard with just the right conductivity for a particular sound. The light degradation for that '90's sound, or the very conductive board for the '50's sound.
If guitarists were more like audiophiles a booming business could be made selling voodoo.

[pjd3]

I guess guitar amp builders should be grateful that they don't have to contend with RF.

An electrical engineer for whom I do a lot of tech work for called me over to see a few scope traces yesterday. One trace was the clock pulses coming directly from the IC pin. The second trace was that same same clock pulse just on the other side of a resistor. Thats it. And a very short trace path. The waveform on the other side of the resistor was a very different shape than the initial clock pulse. Ha, it looked like it has been put in a JCM800 at moderate gain, little spikes, downward and upward curves, etc..

what he was showing me was how a simple resistor at high frequencies becomes an inductor, capacitor as well as a resistor and really does an "EQ" curve thing on the original waveform, which is trying to, or wishes it would remain a sharp cornered pulse wave. I told him he really should have gone into guitar tube amps, that he wouldn't be inconvenienced by such a complexity of issues and distractions. He laughed at me, of course.

So yeah, old fiberboards, lots of components that aren't seen in the schematic. Ha.

Phil D.

[Mark]

I cannot say that I have had a lot of experience with conductive board material, but all my experiences with it have been negative. Typically, noise problems, I built a Rocket prototype on phenolic board that I was given and the amp had a noise issue which I couldn’t find (it didn’t occur to me that the board was the problem), I built a Rocket amp (shown in the Trainwreck discussion area) and it too had the same noise problems, it too was built on a phenolic board which was purchased from RJ this time. Jelle Welagen told me the board was the issue. I managed to take measurements and prove the board was the issue. RJ like a trooper, sends me a Garolite board FOC.

The amp worked perfectly with the Garolite board, it’s hard to say if the two builds sounded different. I used Takman resistors in the second attempt for mojo . I personally think the two builds sounded different, though I could be wrong as it didn’t record anything.

[ChopSauce]

Could you measure something to explain the noise?

NB I'm asking not to confuse you, just for I am not sure about my board. I couldn't measure anything on it but I'm suspecting it anyway ...

[Mark]

Could you measure something to explain the noise?

NB I'm asking not to confuse you, just for I am not sure about my board. I couldn't measure anything on it but I'm suspecting it anyway ...

I don’t have notes on it. I did look at it on a CRO, from memory the noise was similar to having and amp turned up and only a lead plugged in.

Regards

Mark

[WhopperPlate]

I thought of another occasion where old phenolic became conductive . I have a stash of dpdt slide switches like fender style that I inherited from an old shop that I figured were fine, but after chasing gremlins for awhile in a new build I discovered that the switch would literally buzz when I touched it with my finger . Replaced the switch and viola .

[pjd3]

Its not VooDoo.

It is Electrical Alchemy.

PJD3

[TUBEDUDE]

Its not VooDoo.

It is Electrical Alchemy.

PJD3

It's certainly profitable. Tone stones, $400 power cables, and cable lifters each have firm believers. At least people buying clothing and accessories for pet rocks knew it was a scam.

[WhopperPlate]

Its not VooDoo.

It is Electrical Alchemy.

PJD3

It's certainly profitable. Tone stones, $400 power cables, and cable lifters each have firm believers. At least people buying clothing and accessories for pet rocks knew it was a scam.

The scam is the price tag … beyond that it’s just taking fair advantage of the fact that some people can hear a difference , and some people want to hear a difference .

Cable lifters ; I am not gonna dispute the intentional effects… but talk about milking sheep … I had never researched previously…

Hmmm let me think, should I invest my money in a new set of audio transformers , or maybe I should spend it all on cute little carbon fiber stands that I can trip over throughout my house …lol wtf

[WhopperPlate]

Tone stones ; can’t comment there , but you ever see the stone top guitars out there?

There’s a guy who builds them out of Italy I think, can’t remember the name , but they are pretty gorgeous to behold and have a clearly unique sound , though I don’t have any opinions beyond that .