Motorola HS-621 upgrades

[JMicro]

Greetings Ive been into tube radios for years. I am now into tubes amps. I found a nice HS-621 dated 10-1957
Its in great shape except all the resistors and a few caps have been gutted. I bought new metal oxide resistors.
I have the sams and the beitmans. I see beitmans has a 12xa7 version mine is using 12au7 as the inverter.
Is it worth changing it over 2 12xa7? Any ideas on upgrades. I heard its a good idea to change the 5 watt resistor from 220 to 250. Any other improvements? My tube lineup is 6au6 2- 12au7 2- 6v6 and a 5y3
Thanks for any help.

 

[cademan]

Even though the pinouts are the same, the 12AU7 and 12AX7 are not the same. Stick with the 12AU7.

The amplification factor of the 12AU7 is 20. The amplification factor of the 12AX7 is 100.

The circuit is designed for the 12AU7 so if you were to use a 12AX7, you would probably get massive distortion and could damage the circuit.

 

[JMicro]

Thanks since it was basically gutted I was seeing if there was a better circuit design. Ill probably stick with the hs-621 design. Thats how it was originally set up. All the important stuff sockets, transformers and pots are still there. The chassis is very clean otherwise. I only payed 15 bucks for it so I cant complain. It didnt come with any tubes but I have nos to put in. I wish He had left it intact so I knew the layout but with time I will figure it out. They didnt use any terminal strips just mostly unused or daisy chained tube socket terminals.

 

[cademan]

IMHO, I think I would keep it as original as possible. Its a great little 10 watt amplifier and I have heard it in its original record player cabinet which used a 15 inch speaker and a couple of tweeters.

Here's one like the I heard a few years back.

 

[JMicro]

Thats a nice one. I bet it did sound good.

 

[cademan]

Hee, you know that's just a Sam's pic. The one I heard was in a blonde cabinet with some lighter speaker cloth and beat all to hell!