Tag: 4P1L preamp

A new 4P1L Siberian from Martin

Martin kindly sent me a couple of pictures of his latest DHT preamp incarnation of the 4P1L Siberian:

Here are some notes from Martin about his pre-amp:
“A bit more about the circuit, I have built the power supply around the nice AZ1 mesh rectifier. First cap after the rectifier is an obligato 6,8 UF oil followed by a 10H choke and a 100 UF ASC followed by the SSHV2 that I purchased via DiyAudio group buy. On your suggestion I have used the CCS from your 301A preamp. If I refer to my previous preamp with the EF6 penthode in triode connection in parafeed configuration I can say that the 4P1L preamp sounds more detailed and with more air around the instruments. Finally I will do some tests with different output caps to find out if this makes a difference. At the moment I use the Vitamin Q.

Best regards,
Martin”

4P1L with 126C OT load stage

Andy Evans built a pre-amp with the 4P1L and was delighted with the sound of it albeit the 4P1L was running below its optimal operating point: 15mA given the limitations of the 126C interstage transformer.

I went to my workshop to test this configuration and looked at biasing 4P1L with fixed bias and driving it with 1Vrms or more to see what the results were:

So here is the first test at Vg=-4V, Va=74V and Ia=15mA

4P1L test circuit

(all tests were done with the 100k input impedance of the Pete Millett Sound Card interface as the secondary load of the OT. 4P1L had both filaments in parallel and If=600mA)

4P1L test 1

You can see a richer harmonic profile with the OT and distortion is around 0.13% when driven with a 1Vrms providing an expected Vo close to mu (Vo=8Vrms)

The distortion gets very high when output voltage is higher than 9Vrms:

4P1L test 2

 

Now if we bias the valve at a more convenient operating point:

 

We get a slight improvement in THD down to 0.11%. However the distortion above 9Vrms is still high:

4P1L test 3

 

So what if we compare the performance of the OT against the CCS?

4P1L test 5 (CCS)

As we can see from above the distortion is halved. Now if we look at how well this valve could perform if biased in a better operating point, we can see that distortion can be reduce down to 0.03%

4P1L THD 6 (CCS) minimum distortion figure

 

Minimum distortion from a CCS (or gyrator) doesn’t mean that it will sound better. Clearly the OT doubles the THD of the CCS equivalent circuit. Gain here is nearly same on both as OT is in 1:1. Only way of judging both is to do a listening test….

(which is what I’m planning to do next)