#26 pre amp

[Rod Coleman]

Erwin, what's the current rating on the chokes? This will tell you whether shared chokes between L & R channel is possible or not.

Choke-input constructed B+ is definitely recommended.

Well Quasi-choke input anyway. So CLCLC, but with first cap only 0.47uF to 1.5uF (high quality polypropylene, NOT motor-run plastic-housed type). The LCR components PC/HV/S is my favourite in this position.

LCR COMPONENTS|PC/HV/S/WF 470NF 1KV|CAPACITOR, 470NF, 1000V | Farnell United Kingdom

First L should be the 10H (provided this has the highest current rating, preferably 20 - 30mA.

With 20H in the second choke position (rated 10mA+ I hope!) you can used high quality capacitors, without high values.

[milen007]

hi Rob

thanks for the suggestion. i am going for the 0.22uf for the first cap. ill try souce the type you recommend

the chokes are: 20H 125mA (2pcs) and 10H 200mA.

would i better of using them on B+ and ground or dedicated for each channel.

thanks
erwin

[Rod Coleman]

With that current rating, you can try it both ways. I can't be certain which way is best, but if you have noisy mains supply, then 2 chokes in the first stage might be best.

OR you could wire 10H chokes in + and - supply lines, followed by capacitor.

Then, Each channel can have a 20H choke [and a cap] to itself. you get the best of both Worlds!

[merlin el mago]

Finally I purchased one tube AZ1 as rectifier, the transformer for filaments have to be 4VAC & 1.1A?

[boptura]

Originally Posted by merlin el mago

the transformer for filaments have to be 4VAC & 1.1A?

while 1A will be enough, i'd prefer go with a 1.5A minimum.

[andyjevans]

Going back to the chokes, it may be worth looking at the larger picture of preamp plus amp. I don't think separating the channels matters much - better to use the chokes for both channels combined. I don't think you need more than two chokes per supply, but polypropylene caps are important - motor runs are fine.

What I have done is make two HT supplies, each with two chokes. The lower power supply with the choke input AZ1 mesh drives the 26 preamp and the 46 driver on the amp. The higher power supply drives the 300b outputs - this is cap input 4uF and uses the 5R4GY which is the best of all the common rectifier types I've tried, though it does have bigger voltage drop. So that's where my four chokes go!

Andy

[milen007]

hi Andy

thanks for the inputs. really appreciate it. i will try both ways and would take out the extra chokes if two chokes prove to do the same job.

thanks
erwin

[coolzero]

Originally Posted by king4joy

Hi coolzero,

""...Only connection for B+ negative from this rail is 470uf negative to negative (capacitor + to filament's negative and capacitor - to B+ negative)....""

In your diagram, there is already a 470uF capacitor, connected between GND and three resistors.

Did you add one more 470uF capacitor?? If so, could you show it on a diagram please??

Thankyou,

King

The diagram shown is from Jim's site (VT52.com) without a change. What I have now is completely different 26 pre with grid bias. You could find the schematic in this same thread.

[merlin el mago]

Originally Posted by boptura

while 1A will be enough, i'd prefer go with a 1.5A minimum.

It's necessary 2x300V CT 60mA Tx for B+?

It's possible only with 1x240V Tx for B+, if yes how?

[massimo]

Merlin, if you have two diodes only, as in that rectifier, for a full bridge you need a sec with CT or two indipendent secs.
With just one sec, you can implement a hybrid bridge: 1 tube + two ss diodes. See latest andyjevans schematic.

Edit: see post #913