Redesign Sun Audio kit for 45

Hi!

I bought a Sun Audio VT-25 amplifier kit planing to make it into a 45 amplifier.

The new Sun Audio kit did have some upgrade on their parts and chassis.





Later I get to know that the original 6SN7 driver of this VT-25 amp can't really drive a 45 nicely. So I need to find alternatives or redesign the circuit to drive a 45.

Anyone from this forum did this before? Any suggestion ?

Hi Cheah,
   Looks like your hands are itching again!

   A quick calculation suggest that it is able to drive the 45 with the existing same driver of the VT25 SE design. The only modification needed is to reconnect the primary tapping of the OPTx from 8K (VT25) to 5K (45).

  Sun Audio uses the same 6SN7 driver configuration for the 300BSE, 2A3SE and VT25SE kits. The major different of the 3 designs is that the VT25SE uses local feedback (from the power tube), whereas the former 2 designs uses global feedback.

   If you're itching to do something different, you might want to try the Yamamoto Sound Craft A08 circuitry. Drop me a PM if you need the schematic.

  BTW, happy DIY'ing!

Hi Cheah, 

Good to hear that you are onto a new project.  I build a loftin-white 45 (inter-changable with a 2A3) previously (very nice) and found a new home for this amp with a good friend.

In addition, pls note the following differences in operating points :-
i. Filament voltage for vt25/10Y is 7.5v/1.2A whereas a 45 is 2.5V/1.5A.
ii. The operating points for 45 tubes is also possibly lower; B+ at plate = 330V; Ia = 34/36ma. Whereas the VT25 may be 400vDC/18mA.

You may want to try out a different driver tube instead of the 6sn7. 

Maybe a c3g (6.3v/0.3A) in triode mode (gain 40x).  If you like HF extensions, maybe in pentode mode (gain 60x - 80x).

PM/call me if you need further assistance.

Have fun.

Just to add, maybe need to watch-out for specs for the power and filament transformer.  

Per channel, you would require mint 50mA (10mA for the 6sn7, 36mA for the 45 tube + voltage drop resistors).  I like to 'derate' it by a 0.63factor - so to be safe, 80mA per channel.

Conservatively (in stereo) the power transformer/inductors must be able to deliver about 150mA.  

I think the filament transformer should not be an issue.  The 9 VA rating for the VT25 should be adequate for the 45's filament (3.75VA).

VT4C and Mthoi, thanks for your inputs. I think there is no problem on transformers rating to convert the VT -25 amp to 45.
I did it before with my previous VT25 amp. I just change the heater from 7Vdc to 2.5Vac tapping. The OPT also have a 5k primary tapping which I used them for 45.
Thing that I'm comcern here is the voltage swing for 45 would not be ideal if I use the original VT25's driver stage. In order for the 45 poweramp to have maximum output of 2W, the tube need to be biased -56v, 330v at plate with 1500 ohm of cathode resistor. Bias of -56v means I need 112V peak to peak voltage swing from the 2nd 6sn7 stage. The 6sn7 is RC couple to the 45 with only 140v of plate voltage. I'm looking for a better one.

Cheah

Hi Cheah,
   The voltage on the plate of the driver stage (6SN7) should have a reading of around 230V. This is after dropping from 335V via the 27K ohms plate resistor. B+ is about 365V dropping it using a 2.7K ohms resistor for the drivers supply (335V)

   May I know if these are the same values in your VT25 amplifier? Is the power transformer used - SPT150B?

   The same 6sn7 input and driver stage are used for driving the SV300B (apart from the feedback circuitry). It should have the capability driving the 45 too.

Hi VT4C,

For the 45 case, the B+ would be lower as targeted at 330V @ 36mA. 
After the dropping resistor of 2.7k, it would be around 300v. So the 2nd stage plate voltage would be around 235V . But it's plate to cathode voltage is only 165v. This is due to the 2nd stage is direct couple to the first stage.

Hi Cheah,
   Assuming that the second stage of the 6SN7 is able to drive the bias voltage into clipping (which it will)....This means it will have a signal swing of +/- 56. With a plate voltage of 235V; this equates to +291V and +179V repectively.

  With supply voltage of 300V (after the 2.7K resistor), it has a margin (not very much) of 9V...This should be fine. 

  May I know what is the concern on the plate to cathode voltage of 165V?

VT4C, 

Thank you for your clarification here. Although the original 6SN7 driver stage be able to drive the 45 into clipping, I'm trying to look for a driver that have more head room.
I had put in a pair of Duelund 0.22 uf cast Cu in my another Sun Audio VT-25 amp and sounded fantastic. For this 45 amp, I think the amp deserved more.
I'm planing a parelleled 6N7 interstage driving 45. 

Cheah

Hi Cheah,
  You're most welcome.

   Given a choice, this is what I'll do if I'm owning the amp...

1.) Go with a single stage driver rather than cascade in the original Sun Audio design....Lesser is better. I've good experience using triode connect 7788/E810F and D3A. The D3A will have a gain of around 70x. This will provide better headroom. (most Sun Audio amp. does has input sensitivity of 150mV for full output power)

2.) Direct couple and loose the coupling capacitor between the driver stage and 45....Although, this might requires higher B+.

  Yeah!...You're right that the 45 tube deserve more.

If given the space and extra B+, I would use a DHT as driver. But it's all limited in the small amp which I wanted it to be.

Here's my initial plan, 

-Single stage 6N7 which wired in parrelle as  driver.

- Lundahl LL1660/ 10mA wired 1: 1.125 as interstage.

- I will use back original sun audio power and output transformers.

The input sensitivity will be at 1V as the gain of 6n7 is 35. This enable me to match this amp with my DHT preamps.

Cheah

The 6N7 IST 45 amp had completed. Here are some photos.




Inside the amp is crowded with components. As this 45 amp deserved to be more, WBT RCA socket and speaker binding post are being used.


 
The amp is ready to preform.

How does this amp sound?