Ampeg SVT Handwired?

[Patrice B]

For those who don't know. The preamp in actual vintage SVTs is pretty much a rat's nest...and they tend to hum a bit due to ground loops. A hand wired SVT preamp should probably be considered a non-sequitur.

Some parts of my ´74 V4 too are « hand wired » and yeah, I had a bit of troubles chasing a non acceptable level of hum (for me). Hammer on a flea, I had to completely disconnect the preamp out/power amp in wiring and now, it's at an acceptable and non objectionable level. I heard the V4B reissues are church mouse quiet. My 2005 SVT-AV is, BTW, dead quiet.

 

[beans-on-toast]

This is a DIY pre-amp based on the SVP-CL. It uses turret board construction. An interesting build.

DIY bass preamplifier in the Ampeg SVT tradition

 

[agedhorse]

This is a DIY pre-amp based on the SVP-CL. It uses turret board construction. An interesting build.

DIY bass preamplifier in the Ampeg SVT tradition

And a well executed DIY example.

 

[Short4since64]

And a well executed DIY example.

Beautiful layout and construction. A lot of thought and expertise went into this.

 

[Short4since64]

For those who don't know. The preamp in actual vintage SVTs is pretty much a rat's nest...and they tend to hum a bit due to ground loops. A hand wired SVT preamp should probably be considered a non-sequitur.

Agree on the rats nest - and everything is jammed together in the smallest possible space. Hate working on them, but most SVT repairs I have done involved the power amp. Preamp problems are mostly pots and switches.

 

[baxter_x]

but most SVT repairs I have done involved the power amp

Isn't it expectable since the heavy job is done on the power section, not on the preamp?

 

[Short4since64]

Curious if anyone knows for sure - had heard that one design under consideration when the SVT concept was under development was a circuit using two transmitting tubes as outputs with kilovolts on the plates.
Built a 350 watt monster using two 813 tubes at almost 2000 volts while in school. Worked very well, melted voice coils readily. Heavy and scary. Reminded me of the ham operator joke - operator had rebuilt his rig to 10 gallons (10,000 watts) He keyed the mic, the shack crackled with a blue glow, he fell backwards off the chair exclaiming "Oh God!"... YES MY SON. HOW CAN I HELP YOU?

 

[Wasnex]

Curious if anyone knows for sure - had heard that one design under consideration when the SVT concept was under development was a circuit using two transmitting tubes as outputs with kilovolts on the plates.

Could not find the reference from Bill Hughes, but I did find this:

Given the SVT power amp prototype was based on the 811A with a 1500V plate voltage, not a surprise that the first SVT run had another transmitter tube, the 6146B.

Ref: https://www.talkbass.com/threads/70s-svt-bench-test-results.1267078/page-3#post-19699316

 

[agedhorse]

Curious if anyone knows for sure - had heard that one design under consideration when the SVT concept was under development was a circuit using two transmitting tubes as outputs with kilovolts on the plates.
Built a 350 watt monster using two 813 tubes at almost 2000 volts while in school. Worked very well, melted voice coils readily. Heavy and scary. Reminded me of the ham operator joke - operator had rebuilt his rig to 10 gallons (10,000 watts) He keyed the mic, the shack crackled with a blue glow, he fell backwards off the chair exclaiming "Oh God!"... YES MY SON. HOW CAN I HELP YOU?

Not that I ever heard, and I worked under the guy who designed the SVT. The 6146 power tube used in the original SVT was an RF transmitter tube that was rated down through audio frequencies but worked up the 175kHz and was often used for AM transmitters.

Why would 350 watts from the fantasy amp melt voice could readily yet 350 watts be any difference from any other amp that delivered 350 watts?

 

[Hoochie Coochie Man]

Is FR 4 not ubiquitous in modern amplifier construction? I would assume it would be required by common sense and regulatory requirements.

 

[fdeck]

This is categorically incorrect, all Mesa products exported to Europe have been both approved and certified to CB scheme (IEC 60065 and/or IEC 62368) with all national differences listed below. This is right off of the NRTL certification report that's accepted by your own government:

View attachment 5290482

I can't speak for Ampeg, but every product that I have see has carried similar approvals.

Just because you have a difficult time working on an amp doesn't give you the right to make things up. Either get your facts straight or keep them to yourself.

[edit for typo]

In the future, amps will stop getting smaller, as the regulatory labels get bigger.

 

[beans-on-toast]

Could not find the reference from Bill Hughes, but I did find this:



Ref: https://www.talkbass.com/threads/70s-svt-bench-test-results.1267078/page-3#post-19699316

This is what Bill said about his prototype based on the 811A

https://www.talkbass.com/threads/1968-svt-myth-or-reality.1177310/page-2#post-17834594

 

[agedhorse]

Is FR 4 not ubiquitous in modern amplifier construction? I would assume it would be required by common sense and regulatory requirements.

Pretty much so these days with lead free solder (higher soldering temps) and the need for the laminate to be UL94 V-0 flame spread (which makes certification easier, it's not impossible with V-1 but certainly a lot harder depending on where it's used if there's a PIS involved)

In the future, amps will stop getting smaller, as the regulatory labels get bigger.

It's already happened.

 

[Patrice B]

This is what Bill said about his prototype based on the 811A

https://www.talkbass.com/threads/1968-svt-myth-or-reality.1177310/page-2#post-17834594

As I am member #811, I have to ask the experts what this prototype would had given as power rating? Four 811@1500v on the plates, I guess?

 

[beans-on-toast]

As I am member #811, I have to ask the experts what this prototype would had given as power rating? Four 811@1500v on the plates, I guess?

It depends on the design and how it's used.

Data sheet.
https://pentalabs.com/wp-content/uploads/2020/04/811A.pdf

 

[lug]

I used to hand wire lots of stuff. The reason? There were only ever going to be 10 or 20 in existance. Someone conflated "rare" with "quality" and audiophiles have had wet dreams ever since.

 

[Short4since64]

Not that I ever heard, and I worked under the guy who designed the SVT. The 6146 power tube used in the original SVT was an RF transmitter tube that was rated down through audio frequencies but worked up the 175kHz and was often used for AM transmitters.

Why would 350 watts from the fantasy amp melt voice could readily yet 350 watts be any difference from any other amp that delivered 350 watts?

Speakers in 1968 barely carried 30 watts without failing. There were plenty of 500w amps built - the required power to modulate a 1000w amateur AM transmitter - just not many set up as class AB to drive PA speakers. Most were ratty sounding class B. Did this with typical ham components and a mil surplus power supply with mercury vapor rectifiers on a separate chassis. Not in any way practical, but fun for engineering students, and learned how to rewind the secondary of a transmitting modulation transformer to match speaker impedance. SVT design was safe and practical, suitable for other than radio nerds.

 

[agedhorse]

The SVT had the same requirements from speakers of the day as well, that's why the earliest cabinets were intended to be run as 810 cabinet pairs using ~30 watt speakers (on a good day).

 

[Patrice B]

It depends on the design and how it's used.

Data sheet.
https://pentalabs.com/wp-content/uploads/2020/04/811A.pdf

Ok so, theoretically, 320 watts for four tubes@1500vdc on the plates, assuming power supply and preamp design correctly done (things that are way over my head). Not to forget the output transformer obviously… Right?

 

[beans-on-toast]

Ok so, theoretically, 320 watts for four tubes@1500vdc on the plates, assuming power supply and preamp design correctly done (things that are way over my head). Not to forget the output transformer obviously… Right?

Transmitter tubes were rated under specific conditions. Some specs are for intermittent use such as morse code. Designing with them requires understanding their peculiar requirements. The 811A is found in some hi-fi amps.

The Altec 1570B monoblock was rated 165W with a pair of 811A tubes.

[This invalid link has been removed]