Identify year of 1202 164 PT

[friendofotto]

Hi Everybody, I'm a new member here.

I made a purchase of a 1971 JMP 50 watt Lead without doing enough research before buying and I'm thinking that this amp has a replaced Power transformer.

The Transformer has the ink stamp 1202 164 on the bell cover and appears to have the old looking patina.

After doing some research, I learned that these 50 watt stand up transformers can range from approximately 440 vdc down to 350 vdc

The B+ Voltage on mine is 360 vdc.

I saw a YouTube video where a Tech pointed out date code stamps on the side of the bell cover, and took a look at my new to me 1971 JMP50...which has an ink stamp 1 78 at the side of the bell cover.

The output transformer only has the 784 139 ink stamp with no other markings.

Would a 1971 50 watt legitimately have a B+ of 360v ?

Thanks for your help

 

[TAZIN]

Hi Everybody, I'm a new member here.

I made a purchase of a 1971 JMP 50 watt Lead without doing enough research before buying and I'm thinking that this amp has a replaced Power transformer.

The Transformer has the ink stamp 1202 164 on the bell cover and appears to have the old looking patina.

After doing some research, I learned that these 50 watt stand up transformers can range from approximately 440 vdc down to 350 vdc

The B+ Voltage on mine is 360 vdc.

I saw a YouTube video where a Tech pointed out date code stamps on the side of the bell cover, and took a look at my new to me 1971 JMP50...which has an ink stamp 1 78 at the side of the bell cover.

The output transformer only has the 784 139 ink stamp with no other markings.

Would a 1971 50 watt legitimately have a B+ of 360v ?

Thanks for your help

If your PT is stamped 1 78 then it should be from 1978. The 1202-164 for '71 should be around 425v. Double check your heater voltage to ensure your using the correct AC input tap.

 

[friendofotto]

If your PT is stamped 1 78 then it should be from 1978. The 1202-164 for '71 should be around 425v. Double check your heater voltage to ensure your using the correct AC input tap.

Thanks very much.

I'm pretty sure that the amp is set to 120vac. In my area the wall voltage is typically 125, but maybe a bit down with everyone in the area running their air conditioners full blast (me too).

I didn't check the bias current yet either. I'll check when I get home.

From what I've been reading, I appears that 360V B+ is about right for a 1978 50 watter.

Did they always put a date code on the Drake Transformer?
...I ask because I see no date code on the OT

 

[TAZIN]

Did they always put a date code on the Drake Transformer?
...I ask because I see no date code on the OT

No, the date code started in the late '70s I believe. Early versions ('69, '70, '71, '72, etc.) did not have a date code.

I'm pretty sure that the amp is set to 120vac. In my area the wall voltage is typically 125, but maybe a bit down with everyone in the area running their air conditioners full blast (me too).

I know that some of the '72 USA amps that used the 120v tap setting had the heaters only running at 5.8 - 5.9vac and when switched to the 110vac it brought the heaters up to 6.4vac (normal is 6.3vac). This switch also brought the HT voltage up to the 390 - 400vdc range.

 

[stratburst]

I have one with a 75 date code so they started at least in 75. B+ comes in around 360V.

I seen to recall the lower B+ will provide lower headroom. Dial in the heater voltage and see what you think. You may prefer it.

 

[friendofotto]

No, the date code started in the late '70s I believe. Early versions ('69, '70, '71, '72, etc.) did not have a date code.


I know that some of the '72 USA amps that used the 120v tap setting had the heaters only running at 5.8 - 5.9vac and when switched to the 110vac it brought the heaters up to 6.4vac (normal is 6.3vac). This switch also brought the HT voltage up to the 390 - 400vdc range.

Heaters are 6.1vac.
Bias is a bit on the cold side at 10.6 watts
Resistance on the v4 half of the OT is 43.9 ohms
V5 half is 45.4 ohms

It was hard to get a steady reading, but I think that I got 1.3 volts drop across the V4 half, so .029A x 360V looks like 10.66W.

I didn't know about a 110V setting. That's very interesting. I'll need to check it out.

Thanks again