SSS again

[talbany]

The amp I built for Zac Brown had the Super Twin CF driver section in a 124 Non HRM High Plate 100 watt..Being a Nylon picker wanted something real clean he uses a FET/impedance buffer on the floor.. The AT worked OK but I settled on the 12BH7...That CF driver section works fine..

Tony

[crbowman]

Yes, and I saw an odd post stating that the bias supply needs to be -300VDC. I don't understand that.

Yow! Seems like you could bias yourself right into utter silence.
I don't get that either.

I looked over the SVT schematic,I think it may be because the driver is direct coupled with no DC blocking cap. There would be a +DC voltage on the cathode so the -300 is to produce a - potential on the power tube grids.

I'm not really up on why direct coupled would be better than having a cap.

That sounds fairly logical.
Leave it to those wacky Ampeg guys.
I've worked on an SVT before and I don't recall that much bias voltage but it's been awhile. I just remember my back hurting for weeks........

[chris_sanford]

I looked over the SVT schematic,I think it may be because the driver is direct coupled with no DC blocking cap. There would be a +DC voltage on the cathode so the -300 is to produce a - potential on the power tube grids.

I'm not really up on why direct coupled would be better than having a cap.

Direct-coupled drivers (and other stages) eliminate an RC time-constant, and thus reduce blocking distortion, something that is especially important in a bass amp such as the SVT. In addition to biasing the output tube grids, the negative voltage rail can also be used to bias the 'tail' of the driver circuit, which allows for more voltage swing.


HTH,

chris

[txbluesboy]

The amp I built for Zac Brown had the Super Twin CF driver section in a 124 Non HRM High Plate 100 watt..Being a Nylon picker wanted something real clean he uses a FET/impedance buffer on the floor.. The AT worked OK but I settled on the 12BH7...That CF driver section works fine.. Tony

Tony, Can the 12bh7 be plugged into the circuit that is designed for the 12at7 without mods to the circuit?

[talbany]

The amp I built for Zac Brown had the Super Twin CF driver section in a 124 Non HRM High Plate 100 watt..Being a Nylon picker wanted something real clean he uses a FET/impedance buffer on the floor.. The AT worked OK but I settled on the 12BH7...That CF driver section works fine.. Tony

Tony, Can the 12bh7 be plugged into the circuit that is designed for the 12at7 without mods to the circuit?

I drew out the load lines when I did the mod, and it looked like the bh7 curves were more linear than the At7,, the Bh7 can run at double the current than the At7 can, making it a more powerful driver tube but the AT has more gain 60% vs 30% w 12bh7..In this case your not really looking for gain so much as current to drive the grids as well as linearity..They both have max plate voltage of 300v.. It's been awhile since I did that one and couldn't find anything in my notes on the amp to suggest a tweak there.. So I would say they are interchangeable but can't say for +...Both will work I just remember liking the BH a bit better... Good Luck

[David Root]

The bias supply may come off a -300VDC node but what goes on the tubes is a different matter. I worked on two SVTs but not recently. I do remember they are biased at 72 mA each set of three tubes on a side, (cathode current) so 24 mA each tube.

[Bob-I]

Tons of great information about drivers, thx guys!!

I need to plan for some experimentation here. Unfortuantley the Peavey doesn't have a tap I can use to get a -300V for the direct coupled bias.

[thyx]

Some SSS seem to be using the 12BH7 ampeg section.

Yes, and I saw an odd post stating that the bias supply needs to be -300VDC. I don't understand that.

150 volts will do. You don't need 300. Old Music Man trannies put out 650 volts B+...the SSS runs at around 500 volts at the plates, so all you need do is take the 150 volts off the bottom of the PT. That's what Dumble did...though there are other ways (such as a seperate PT for the bias).

[glasman]

Using the Marshall 50W method for developing bias voltage could be used to get the needed negative voltage source. The only issue that might occur is an imbalance on the secondary of the transformer due to the draw from the 12BH7.

SVTs used -190V for their rail, but they also have a secondary winding to development that supply.

Gary

[Bob-I]

All great stuff. I'm learning a ton about drivers and bias here.

The Peavey PT has a 56V tap for bias. If I run a voltage doubler from that tap I might come close, according to the Duncan PSU designer I'll get 130V.

I CAN get a - voltage from a doubler right?

[talbany]

The only issue that might occur is an imbalance on the secondary of the transformer due to the draw from the 12BH7.

SVTs used -190V for their rail, but they also have a secondary winding to development that supply.

Gary[/quote]

I second that .. The Marshall method on some builds can get mushy and develop quite a lag time to recover due to the in-balance.. This can effect tone greatly especially when playing at loud volumes for long periods.. Your bias doesn't recover as quickly so the tubes run hotter(or colder bias however you look at it)..This maybe more of an issue with the current hungry BH... You can speed up the recovery time/constant by going with smaller say .022 grid couplers as well as 100K bias feed resistors instead of the 200k's..Also fast reacting diodes in the bias supply will also help some.. Doublers can also have an effect on the lag time..Just be aware

Tony

[Blindog]

All great stuff. I'm learning a ton about drivers and bias here.

The Peavey PT has a 56V tap for bias. If I run a voltage doubler from that tap I might come close, according to the Duncan PSU designer I'll get 130V.

I CAN get a - voltage from a doubler right?

I agree I'm doing somethin similar with a KMD for a donor.
I've got a 50V tap, and I have a 32-0-32 secondary that was used for the opamp supply. Wonder if that could be used with a full wave bridge and get close. I looked at Duncan's PSU program, but I couldn't find a generic supply with a CT to play around with.

Great thread!

Mark

[tictac]

I think you'll find the voltage doubler circuit won't be able to supply enough current for the direct coupled circuit. You'll need to add a transformer to supply the negative bias if your PT doesn't have the winding for it.

I had this problem with my SSS build and settled for the Fender UL Twin type circuit with the added cap on the cathode. It still sounds good to me but I would have liked to gone with the direct coupled too, just didn't have the room for another transformer.

TT

[Structo]

Interesting KMD amp there.

From what I remember they were mostly MOSFET amps.
Looks like you got lucky and got the all valve one.

Is it the XV100SD?

Kind of weird it only has 170v secondaries.

[Blindog]

Interesting KMD amp there.

From what I remember they were mostly MOSFET amps.
Looks like you got lucky and got the all valve one.

Is it the XV100SD?

Kind of weird it only has 170v secondaries.

Yup, XV100SD, mid '80's I think.Sambora was supposedley using(or endorsing) them then. It sounded just ok. I'm just keeping the power supply and output circuit.

Mark