peavey Iron

[norburybrook]

I'm just about to buy a cheap peavey triumph 120w combo for parts.

It has working

PT
OT 4 and 8 ohm tap
reverb TF
reverb tank
black widow 12 inch speaker

I was thinking along the lines of a SSS or small special with reverb.

I've looked at the schematic and it seems the PT is putting out 500vDC on the plates of the 6L6's. which obviously they can take , but I was wandering about the general voltages. Would the dropping string resistors be enough to get this into SSS territory or would I need to use another way to reduce the voltages



TIA


Marcus

[Stevem]

Sorry, but I need to ask what is SSS short for, solid state super?

[norburybrook]

Dumble Steel string Singer



It's the 100w Dumble clean amp SRV and John Mayer use.




M

[larsvictor]

Marcus,

in the original #002 circuit there is the "PT driver gain stage". This is a normal 12AX7 valve. Plate voltage is taken from the screen resistor node (~450 VDC). Crucial in that part of the circuit is that the grid and cathodes (it´s designed as a cathode follower) have a negative potential of ~ -50 VDC. So the plate - cathode voltage is 500 VDC ! That exceeds the allowed voltage a lot.
With 500 VDc on your PT plates your screens will have just a little less. This will be even more stress to that PT driver tube.

TAG members reported trouble in that gain stage with a tube "survival" of only 2 weeks. Others had no problems. Of course you may take a lower voltage node or try a different tube.

Martin Manning designed a totally different approach for that tube:
https://tubeamparchive.com/viewtopic.ph ... 489#361489

Lars

[martin manning]

Being a clean amp, I think 500V on the plates might be just fine.

[norburybrook]

Lars,

thanks I need to look at that more closely.

I was thinking about Mhartmans SSS

https://tubeamparchive.com/viewtopic.php?t=29722

or the AN Dumbleland reverb.


I need to check if the AN has that cathode follower as part of the circuit

https://tubeamparchive.com/viewtopic.php?t=29774



I'm now moving into unchartered territory for me.

A donor amp with certain specs rather than buying the 'correct' parts for a build.

I suppose I need find a way of eith reducing that cathode follower voltage or find an amp that would work with the Peavey Iron.

cheers

marcus

[norburybrook]

ah..I've opened up a can of worms now looking at that thread about the cathode follower!!!!


Ok, Sound wise , what does the cathode follower do in comparison with the (I presume) non cathode follower of a small special or Jm sig amp?

so really , I'm asking; if it's going to be difficult to get that iron to work for a SSS, would I be losing something by building the small special?AN wonderland with reverb?



M

[larsvictor]

MHartmann´s has the cathode follower too.

The AN Wonderland has a common power amp circuit.

A cathode follower is a current gain stage. IMO it avoids overdriving the power tubes and keeps the sound "clean". I´m shure here are guys who can explain that in detail.

[norburybrook]

Marcus,

in the original #002 circuit there is the "PT driver gain stage". This is a normal 12AX7 valve. Plate voltage is taken from the screen resistor node (~450 VDC). Crucial in that part of the circuit is that the grid and cathodes (it´s designed as a cathode follower) have a negative potential of ~ -50 VDC. So the plate - cathode voltage is 500 VDC ! That exceeds the allowed voltage a lot.
With 500 VDc on your PT plates your screens will have just a little less. This will be even more stress to that PT driver tube.

TAG members reported trouble in that gain stage with a tube "survival" of only 2 weeks. Others had no problems. Of course you may take a lower voltage node or try a different tube.

Martin Manning designed a totally different approach for that tube:
https://tubeamparchive.com/viewtopic.ph ... 489#361489

Lars

Why can't the voltage be taken from a different i.e lower 'node' further down the dropping string B+ chain?


M

[norburybrook]

MHartmann´s has the cathode follower too.

The AN Wonderland has a common power amp circuit.

A cathode follower is a current gain stage. IMO it avoids overdriving the power tubes and keeps the sound "clean". I´m shure here are guys who can explain that in detail.

Ah so without it the amp might break up a little. I might prefer that even

M

[larsvictor]

Why can't the voltage be taken from a different i.e lower 'node' further down the dropping string B+ chain?

Shure. TAG member lovetone did it that way...

[martin manning]

Same problem as bcmatt had converting a Traynor, you have no negative outage available with a bridge rectifier.

[norburybrook]

Ah..so the wonderland it is then which I presume is a kind of small special with reverb.

Am I just building a fender twin here ?


Anyone have any other ideas about what to build with this set of iron?

I suppose I could build another ODS but add reverb.

Marcus

[norburybrook]

Same problem as bcmatt had converting a Traynor, you have no negative outage available with a bridge rectifier.

OK, I'm right in saying then that on the power board of the SSS there's a node that comes straight form half the main AC transformer voltage, it then goes through the 3.3k and a single diode to rectify it for the negative DC voltage needed for the cathode follower and that's where me having 500v will be an issue?

how much voltage drop can safely be found using resistors or diodes?

is there no way of dropping down from 500vDC to the required 460vDC just using large wirewound resistors? and wouldn't that then solve the problem?


Marcus

[martin manning]

But you can always add the small extra transformer for the CF power supply, or perhaps use the Peavey's bias winding (looks like it is separate from the HT) and a voltage tripler to get the negative voltage. You could then use the PI node for the driver's positive voltage.