Ampeg SVT 1977 plate resistors and filter cap replacement...

[Domlamp]

Hi everyone! I have here a SVT bass amp from 1977 and will have to do some update and need help for replacing parts:

-All plate resistors (originally 5 ohms 5W ceramic resistors) have been changed in the past (one is 8 ohms 10W, some 5 ohms 5w...) and i read that change them for wirewound non inductive 10 ohms 5 W 1% is the best (much more expensive though...) But why 10 ohms? On the schematics, it's 5.1ohms 5W. Here at the local electronic store, they have 4.7 Ohms 5W standard ceramic resistor. Will they can make it if i replace all 6 by those? or better with the 10 Ohms 5W 1%? here's a link to see:

https://www.mouser.ca/ProductDetail/71-NS5-10?R=NS00510R00FB12virtualkey61300000virtualkey71-NS5-10

-One of the original filter cap (Mallory 90Uf 450V) show signs of wear (bumps on each side of electrolytic) and will have to be replace. A Sprague Atom 100Uf 450V will be OK in that position? here's a link:

https://www.mouser.ca/ProductDetail/Vishay-Sprague/TVA1718-E3?qs=%2BMk1bzH0xpu2wa0JpUGhSA==

Thanks!

 

[beans-on-toast]

It doesn't matter if the plate resistors are 8 ohms, 5 ohms, 10 ohms, or 5.1 ohms. All six should be the same with a balanced set of power tubes. The specification says 5W, use that not a higher wattage resistor. You want it to blow open if there's a problem.

Ampeg does the following in their current SVT's. I follow their lead.

Rather than use a plate resistor install this ferrite bead. It helps protect the amp from high frequency oscillations. If you want to keep plate resistors, install the beads in series with the resistors.

Link Removed

If you have the diodes in parallel with the screen resistors, remove them. They are useless. Ampeg does not install them.

Change out the 22 ohm 6550 screen resistors for 220 ohm, metal film, 1/2W, flameproof, fusible (this means that it acts like a fuse).

This is what I use, mouser should have an equivalent.

Invalid Link Removed

A 100uF TVA cap is commonly used as a replacement in these amps.

Also inspect C23 in the preamp, the 40uF@450V cap. It often needs to be replaced.

 

[Domlamp]

It doesn't matter if the plate resistors are 8 ohms, 5 ohms, 10 ohms, or 5.1 ohms. All six should be the same with a balanced set of power tubes. The specification says 5W, use that not a higher wattage resistor. You want it to blow open if there's a problem.

Ampeg does the following in their current SVT's. I follow their lead.

Rather than use a plate resistor install this ferrite bead. It helps protect the amp from high frequency oscillations. If you want to keep plate resistors, install the beads in series with the resistors.

Link Removed

If you have the diodes in parallel with the screen resistors, remove them. They are useless. Ampeg does not install them.

Change out the 22 ohm 6550 screen resistors for 220 ohm, metal film, 1/2W, flameproof, fusible (this means that it acts like a fuse).

This is what I use, mouser should have an equivalent.

Invalid Link Removed

A 100uF TVA cap is commonly used as a replacement in these amps.

Also inspect C23 in the preamp, the 40uF@450V cap. It often needs to be replaced.

Thanks!!! Why Ampeg use plate resistors? Ferrite is out at Mouser actually... I think i'll go with the 4.7 Ohms 5W ceramic that the local store have here if that's ok. I'll definitively replace the C23 too! Thanks!

 

[beans-on-toast]

Thanks!!! Why Ampeg use plate resistors? Ferrite is out at Mouser actually... I think i'll go with the 4.7 Ohms 5W ceramic that the local store have here if that's ok. I'll definitively replace the C23 too! Thanks!

Just make sure that the resistors fit on the PCB space.

Initially the amp was designed with the plate resistors, they limit current flow in the event that a tube shorts. Later ampeg changed the design and for some time have been using the beads. They do a good job in protecting the amp. Normally the resistance is low, if a high frequency oscillation occurs, the beads resistance increases, this protects the amp.

If it isn't too late, you can contact mouser and ask if they have an alternate part that is equivalent. A few companies make them under different part numbers.

 

[Domlamp]

Many thanks! No, it's not too late as this amp is a restoration project for a friend (see my other thread with the 2 x SVT cab to restore... A nice one!) So, i'll maybe check at digi-keys or other place like that.

I'm back from the local electronic store. They were mismatched with their resistors and only have 47 Ohms (instead of 4.7 Ohms...) If i can't find the beads, i think i'll go with the wirewound 10 Ohms 5W non inductive resistors.

I've also notice the 6 diodes on the screens... So, get all out of there and replace by a 220 Ohms flame retardent (fuse type) resistor? Simply as that? I just wonder (again! I'm more into the Fender and Marshall guitar amps...) why a diode in parallel with a resistor was placed there? Thanks for your time!! I appreciate a lot!

 

[beans-on-toast]

Many thanks! No, it's not too late as this amp is a restoration project for a friend (see my other thread with the 2 x SVT cab to restore... A nice one!) So, i'll maybe check at digi-keys or other place like that.

I'm back from the local electronic store. They were mismatched with their resistors and only have 47 Ohms (instead of 4.7 Ohms...) If i can't find the beads, i think i'll go with the wirewound 10 Ohms 5W non inductive resistors.

I've also notice the 6 diodes on the screens... So, get all out of there and replace by a 220 Ohms flame retardent (fuse type) resistor? Simply as that? I just wonder (again! I'm more into the Fender and Marshall guitar amps...) why a diode in parallel with a resistor was placed there? Thanks for your time!! I appreciate a lot!

Yes, remove the 22 ohm resistors and the diodes, replace the 22 ohm with the 220 ohm resistors.

 

[Wasnex]

Many thanks! No, it's not too late as this amp is a restoration project for a friend (see my other thread with the 2 x SVT cab to restore... A nice one!) So, i'll maybe check at digi-keys or other place like that.

I'm back from the local electronic store. They were mismatched with their resistors and only have 47 Ohms (instead of 4.7 Ohms...) If i can't find the beads, i think i'll go with the wirewound 10 Ohms 5W non inductive resistors.

I've also notice the 6 diodes on the screens... So, get all out of there and replace by a 220 Ohms flame retardent (fuse type) resistor? Simply as that? I just wonder (again! I'm more into the Fender and Marshall guitar amps...) why a diode in parallel with a resistor was placed there? Thanks for your time!! I appreciate a lot!

AFAIK, The diode is intended to protect the screen resistor when the amp is pushed into overload. It's actually fairly common for the screen resistors to open when a tube fails, and IMHO this is a good thing because it cuts a faulty tube off and prevents other more serious damage from occurring. With the diodes in the circuit, I believe it's more likely that solder traces will be destroyed as a result of high screen current. Also if the screen resistor doesn't open, then if you're lucky the plate resistor burns. Better to fry a few resistors than damage the circuit board or fry one of the transformers. Resistors only cost a few cents.

AFAIK, Changing the screen resistors to 220 ohms is supposed increase the reliability of the tubes, as it limits screen current.

 

[Domlamp]

AFAIK, The diode is intended to protect the screen resistor when the amp is pushed into overload. It's actually fairly common for the screen resistors to open when a tube fails, and IMHO this is a good thing because it cuts a faulty tube off and prevents other more serious damage from occurring. With the diodes in the circuit, I believe it's more likely that solder traces will be destroyed as a result of high screen current. Also if the screen resistor doesn't open, then if you're lucky the plate resistor burns. Better to fry a few resistors than damage the circuit board or fry one of the transformers. Resistors only cost a few cents.

AFAIK, Changing the screen resistors to 220 ohms is supposed increase the reliability of the tubes, as it limits screen current.

Thanks! So, if i understand correctly, it's OK to remove the diodes and replace the 22 Ohms screen resistor by a 220 Ohms ONLY (no diodes with it)? Diodes was there only to protect the screen resistors? So... Why install a protection for something that HAVE to fry in case of a bad tubes or anything else?

 

[agedhorse]

Non-inductive resistors are not necessary.

 

[Domlamp]

Non-inductive resistors are not necessary.

thanks! As i said, i'm more in the guitar tube Marshall-Fender amps... Screen resistors are much higher in values (ex: 1k 5W for 6CA7 on screen in a JMP), why in a SVT, only 220 Ohms 1/2w (in the case i drop off the 22 Ohms 1/2w with his parallel diode) for protection on screens?

 

[Wasnex]

thanks! As i said, i'm more in the guitar tube Marshall-Fender amps... Screen resistors are much higher in values (ex: 1k 5W for 6CA7 on screen in a JMP), why in a SVT, only 220 Ohms 1/2w (in the case i drop off the 22 Ohms 1/2w with his parallel diode) for protection on screens?

I think this is the part you may be missing:​

Guitar amps tend to run the screens just a few volts below the plates. Because of this, the screens would pull lot's of current and exceed their dissipation rating if not for the relatively large screen resistors. The higher the current passing through the screen resistors, the more voltage they drop.

The SVT has a dual rail power supply so the plate voltage is up around 700V and the screens are significantly lower at around 350V. Since the screen voltage is way lower than the plate voltage, the screens do not try pull as much current. I believe if you make the screen resistors too large it will limit the amp's ability to make power.

 

[agedhorse]

Different tube type and (very) different circuits.

 

[Domlamp]

I think this is the part you may be missing:​

Guitar amps tend to run the screens just a few volts below the plates. Because of this, the screens would pull lot's of current and exceed their dissipation rating if not for the relatively large screen resistors. The higher the current passing through the screen resistors, the more voltage they drop.

The SVT has a dual rail power supply so the plate voltage is up around 700V and the screens are significantly lower at around 350V. Since the screen voltage is way lower than the plate voltage, the screens do not try pull as much current. I believe if you make the screen resistors too large it will limit the amp's ability to make power.

Wow! Many thanks for the clear explanation! My first SVT! As Agedhorse said, it's way different from a guitar amp! Thanks again!! Dominic

 

[SethRiff]

If you parallel multiple tubes they tend to oscillate at high power/ high frequency

To fix the issue you add 47 to 100r resistors on the plates

Most the oscillation is caused by lead length or trace length
which forms stray inductance/capacitance and causes the oscillation problem.

10 ohms was the original plate resistor value for a SVT probably why your recommended
a 10 ohm value. Which actually needs to be higher.

SLM tried to cure oscillation issues by placing the tubes closer together.
to reduce the trace lengths. Then they added ferrite beads L1- L6 on the power board.

Low inductance resistors are recommended since the actual problem is inductance.
Your trying to reduce the problem not increase it.

 

[Domlamp]

If you parallel multiple tubes they tend to oscillate at high power/ high frequency

To fix the issue you add 47 to 100r resistors on the plates

Most the oscillation is caused by lead length or trace length
which forms stray inductance/capacitance and causes the oscillation problem.

10 ohms was the original plate resistor value for a SVT probably why your recommended
a 10 ohm value. Which actually needs to be higher.

SLM tried to cure oscillation issues by placing the tubes closer together.
to reduce the trace lengths. Then they added ferrite beads L1- L6 on the power board.

Low inductance resistors are recommended since the actual problem is inductance.
Your trying to reduce the problem not increase it.

Thanks! So, higher plate resistors in that case or like said earlier)

If you parallel multiple tubes they tend to oscillate at high power/ high frequency

To fix the issue you add 47 to 100r resistors on the plates

Most the oscillation is caused by lead length or trace length
which forms stray inductance/capacitance and causes the oscillation problem.

10 ohms was the original plate resistor value for a SVT probably why your recommended
a 10 ohm value. Which actually needs to be higher.

SLM tried to cure oscillation issues by placing the tubes closer together.
to reduce the trace lengths. Then they added ferrite beads L1- L6 on the power board.

Low inductance resistors are recommended since the actual problem is inductance.
Your trying to reduce the problem not increase it.

Thanks! From the schematics, it's 5.1 Ohms, but how much higher can the plate resistors can be? Maybe it's a "try and listen" job, but with the weight of the chassis... So, my problem for now it's that Mouser have the ferrite, but not the 220 Ohms screen resistors and Digikeys have the 220 Ohms, but not the ferrite.... I'm a bit stuck! If i have to order at one place, can i have a good compromise? Let's say:

-Plate resistors of 56 Ohms 1/2 W + 220 Ohms fuse type resistors for screen if i have to order from Digikeys.
-Ferrite Beads for plate (only) + another screen resistor that can do the job if i order from Mouser.

What's the best deal? Oh God...

 

[SethRiff]

1977 SVT doesn't have ferrite beads. It was a poor attempt by SLM to fix oscillation
in the 90s versions of the SVT.
Don't worry about it at all.


There is nothing to listen to it doesn't change the sound at all.
It helps reduce oscillation.

The original plate resistors for a SVT were 10 ohms to 5 ohms.
They could have solved the problem if they were raised to 68 ohms.

The screen resistor values never had to be raised. Screens blow up because the amp oscillates at high power.

Its good practice to yes replace the power resistors in a SVT power section because the values drift.
All of them are 5 watts. To get a accurate plate current measurement the 1 ohm cathode resistor
should have no drift either.

Original Values

Plate = 5 to 10 ohms 5 watt
Screen = 22 ohm 5 watt
Cathode = 1 ohm 5 watt

All you need to buy is 68 ohm 5 watts and 1 ohm 5 watts.
Non inductive, 1 to 3% tolerance install like this =

Plate = 68 ohm 5 watt
Screen = 68 ohm 5 watt
Cathode = 1 ohm 5 watt.

2 values, literally fixes most the problems and ridiculous changes over years to fix a simple problem

 

[agedhorse]

If you parallel multiple tubes they tend to oscillate at high power/ high frequency

To fix the issue you add 47 to 100r resistors on the plates

Most the oscillation is caused by lead length or trace length
which forms stray inductance/capacitance and causes the oscillation problem.

10 ohms was the original plate resistor value for a SVT probably why your recommended
a 10 ohm value. Which actually needs to be higher.

SLM tried to cure oscillation issues by placing the tubes closer together.
to reduce the trace lengths. Then they added ferrite beads L1- L6 on the power board.

Low inductance resistors are recommended since the actual problem is inductance.
Your trying to reduce the problem not increase it.

Hold on a minute, what do you think the ferrite beads are for?

They INCREASE the source inductance at high and RF frequencies. This is why low inductance resistors are not needed.

Plate resistors are used for other purposes as well.

 

[agedhorse]

1977 SVT doesn't have ferrite beads. It was a poor attempt by SLM to fix oscillation
in the 90s versions of the SVT.
Don't worry about it at all.


There is nothing to listen to it doesn't change the sound at all.
It helps reduce oscillation.

The original plate resistors for a SVT were 10 ohms to 5 ohms.
They could have solved the problem if they were raised to 68 ohms.

The screen resistor values never had to be raised. Screens blow up because the amp oscillates at high power.

Its good practice to yes replace the power resistors in a SVT power section because the values drift.
All of them are 5 watts. To get a accurate plate current measurement the 1 ohm cathode resistor
should have no drift either.

Original Values

Plate = 5 to 10 ohms 5 watt
Screen = 22 ohm 5 watt
Cathode = 1 ohm 5 watt

All you need to buy is 68 ohm 5 watts and 1 ohm 5 watts.
Non inductive, 1 to 3% tolerance install like this =

Plate = 68 ohm 5 watt
Screen = 68 ohm 5 watt
Cathode = 1 ohm 5 watt.

2 values, literally fixes most the problems and ridiculous changes over years to fix a simple problem

Again, you are making assumptions that are not entirely accurate.

A 68 ohm plate resistor is awfully high for a 6550/KT-88 type tube, the screen resistance is a function of what the primary purpose of the screen resistance is being used for. Different values are compromises for different parameters, since the Vscreen supply is about 1/2 of the Vplate supply, the primary purpose is as a fuse-able element in the event of tube failure. It also serves to provide some sag (negative feedback) as the screen current wants to increase under heavy load.

With the values used, there should be no aging drift in these resistors under normal operation because the parts are used well within their margins. They can drift as part of their fuse-able function under abnormal (failure) conditions.

 

[agedhorse]

As a follow-up, under high drive conditions, a 6550 can support plate currents of 300mA. This is 1/2-wave in a class AB amp, so the average power dissipation is 1/2 of the 1/2-wave calculation.

This would equate to a Pd at the 68 watt plate resistor of over 6 watts 1/2-wave and 3 watts average full wave. That is a lot of loss per tube (6 watts) which is why it's not often seen (I can't recall any commercial designs using this high of a plate resistor)

Using 100 ohm plate resistors would be absurd, the losses would be ridiculous and would be pushing the limits of a 5 watt resistor as well.

There are other ways to handle the stable paralleling of tubes.

[disclaimer - I worked under Bill Hughes (designer of this particular circuit) while he was the director of R&D at Fender]

 

[Domlamp]

Hold on a minute, what do you think the ferrite beads are for?

They INCREASE the source inductance at high and RF frequencies. This is why low inductance resistors are not needed.

Plate resistors are used for other purposes as well.

ok! So, beads or not beads, that's the question!! ahah!