Re channels and biasing on '72 PA20

[surfguy13]

Hi guys

Greetings from the UK!

Just acquired my second PA20 head and have a couple of questions.

This head is a '72 and has never been messed with.....pretty much mint and it sounds great. The quietest head I've ever used. It's just been serviced by Marshall and the only work carried out was 4 new vol/tone pots. It's currently wired to 8 ohms.

Anyway to my ears the 2 'channels' sound identical which I'm guessing would be the case for a PA amp. Can anyone confirm this? I never jumpered my last PA20 ('70) for this reason.....couldn't see the point of mixing two channels that sound identical. However, be good to know if the two channles are the same and whether anyone has a view on the benefits (or lack) of jumpering with these amps.

The second question is re biasing.....I seem to remember asking this question when I had my last PA20 and got some great advice but I honestly can't remember what the consensus was.

This head is fitted with two Boogie EL84s and a pair of Boogie ECC83s. Sound excellent but have no way of testing them. I believe that I was advised that if the valves test within the parameters of the fixed bias it's OK but......if they don't?!!

Any advice and/or links would be gratefully received.

Finally (yes, this is a bit of a loaded post!! ).....the previous owner asked Marshall to change it to 8 ohms from 16. They warned him that this would cause a drop in volume. Is this the case? I would prefer to run it at 16 ohms as my cabs are 16 but can obviously re-wire to 8 in a flash. Happy to change it back to 16 but not sure what the pros and cons are?

Few photos:

 

[slide222]

a drop in volume wasn't mentioned and the EL84 pair were brought as a matched set -they were in the same box

 

[surfguy13]

Hi Pete

I reckon my brain is deteriorating fast!! I could have sworn you told me that Marshall had said that changing from 16 ohms to 8 ohms would result in a drop in volume? I must have been dreaming!!!

Yeah, I thought the Boogie EL84s were a matched set and they really so sound nice. The amp just sings.

Trying to decide what cabs to use it with....the G12Hs in the cab I brought over sound perfect so maybe I'll just stick to that cab as it's 8 ohms and leave it at that!

a drop in volume wasn't mentioned and the EL84 pair were brought as a matched set -they were in the same box

 

[slide222]

got my jmp1h and jcm1h today , and hopefully the little splitter will arrive tomorrow, and some various leads to complete the setup

 

[LMP2204]

Jump the channels and see what happens. set one channel bright and set the other darker. see if it increases any gain. sound clips would be sweet!

 

[slide222]

you can vary the sound using a jumper by setting each channel different, basicly wot LMP2204 said. I use to set the first flat and play with the 2nd channel

 

[surfguy13]

Thanks Pete and also LMP2204.......!

Right, set to this morning and played the amp for about 20 minutes on channel 1....then jumpered it with channel 2, with the setting on both channels the same, and the increase in gain was astounding.

I was using an old mid 60s tele straight into the head and then into a 2x12 with '72 G12Ls. With the vol at 2 o'clock going into channel 1 and not jumpered the guitar was lush but not breaking up, even with the guitar vol up full. Leaving everything the same and jumpering the channels the guitar was breaking up REALLY nicely with, it seemed, more harmonics. Jumpering made a really massive dfference.

With the same guitar I then plugged the head into a 2x12 loaded with '75 G12Hs and needless to say the guitar was breaking up much earlier than with the G12Ls. But, bringing the volume on the amp back to 11 o'clock with the guitar vol up full things were just starting to break up. The same thing jumpered resulted in a huge boost and I was getting really thick creamy drive......and this is an old tele I remind you.

So, I shall be using this amp jumpered permanently from here on in!!!!

 

[mickeydg5]

Hello Surfguy13

Here are the answers to your questions if still needed for the model 1917 PA 20 watt.
#1 The two channels are identical.

#2 The power amplifier with EL84s is cathode or self biased. There is no manual bias adjustment.

#3 Any tap being 16, 8 or 4 ohms from the output transformer will produce the same volume.

 

[surfguy13]

Hey mickeydg5

Thanks so much for confirming that.....really appreciate it.

Hello Surfguy13

Here are the answers to your questions if still needed for the model 1917 PA 20 watt.
#1 The two channels are identical.

#2 The power amplifier with EL84s is cathode or self biased. There is no manual bias adjustment.

#3 Any tap being 16, 8 or 4 ohms from the output transformer will produce the same volume.

 

[Georgiatec]

got my jmp1h and jcm1h today , and hopefully the little splitter will arrive tomorrow, and some various leads to complete the setup

Then all you need is a Bad Cat Unleash....trust me, you do.

 

[slide222]

well I just had a look on ebay and its there . no I don't need 1 of those ... that's why I have this set up . thanks anyway