Adapted 5F2A with EZ80 and EL84

[stephenhartley]

My son and I have just finished building a SE 5 watt amp using a Princeton 5F2A circuit, but we've swapped the 5y3gt rectifier for an EZ80 and the 6V6 for an EL84 (we're in the UK).

we've compensated by swapping the cathode resistor from 470R to 150R

The tone at low volume is sweet, but on turning up it breaks up into into a brittle harsh distortion.

Any suggestions on how to improve the sound please?

[The Dude]

Schematic?

[stephenhartley]

Schematic?

It's just the Princeton 5F2A but with the 5Y3GT replaced with an EZ80 and the 6V6 replaced with and EL84. The 470R cathode resistor is replaced with a 150R

[Dave H]

The tone at low volume is sweet, but on turning up it breaks up into into a brittle harsh distortion.

Any suggestions on how to improve the sound please?

You probably don't want to hear this but... I used to gig with a 2 x EL84 push pull amp and I liked the sound of it cranked up so I bought a Valve Junior head thinking I could get the same sound at a lower volume at home but It had that harsh distortion. I eventually changed it to 6V6 and it's fine now.

A couple of less drastic suggestions.

It will have more negative feedback with an EL84 try doubling the value of the feedback resistor or disconnect it to see (hear) if that helps to tame the harsh breakup. What is the EL84 plate voltage? It may sound smoother at a lower voltage.

[Justin Thomas]

Paging Chuck, the EL34 Master himself...

Justin

[Chuck H]

Having worked over a few Champs (similar enough to a Princeton) and having worked with a few el84 builds (but never SE el84)...

Champs, IMHE, often have harsh distortion. I haven't been able to isolate all the causes, but I have mitigated it many times. The first thing I would suggest would be a shunt filter (conjuctive filter?). A Zobel network across the OT primary. Those hashy spikes don't stand a chance. The tricky part would be choosing a capacitance that doesn't screw up the clean tone. And don't be afraid to make the resistance a little lower than typical for this circuit (which I've read as 1.4x primary impedance). The last Champ I "corrected" used a 2kV 1500p cap straight across the OT.

The next thing I would mention is that it's easy to over dissipate the screens on el84's. They're not as burly as 6v6's. If the screen is attempting to dissipate power instead of the plate it's both bad for the tube and harsh sounding. Perhaps add a 1k/3W screen grid resistor.

I have also noticed that the right speaker can help alot. Also, most stock Champ OT's (likely the same OT's sold for the 5F2A?) have really poor LF when you pound on them. The frequency balance favoring HF doesn't help with harshness. Using a bigger OT has helped a lot for me. But these are expensive components so certainly try the above first.

JM2C

EDIT: Oh! Also,.. 150 ohms for the cathode resistor may not be hot enough. If the clipped wave form is too wonky it can show a bad spike on the favored swing. What is your plate voltage and what is the voltage on top of the cathode resistor?

[Leo_Gnardo]

stock Champ OT's (likely the same OT's sold for the 5F2A?) have really poor LF when you pound on them. The frequency balance favoring HF doesn't help with harshness. Using a bigger OT has helped a lot for me.

Deafinitely true! If you start with a dinky stock Champ OT, leave some space on the chassis so you can swap in a bigger better SE OT as an upgrade. Also connect your speaker with quarter inch jack & plug so you can opt to drive other speaker cabs with an ordinary cable. You'd be surprised how good a SE Princeton or Champ can sound through bigger speakers, even a 4x12.

Couple of weeks ago I had a Laney SE combo in with a blown OT. I happened to have a Ceria SE transformer left over from a conversion (SE to PP) project a couple years earlier. Customer & I were both impressed. Him, more so because he got to compare the results before & after the OT swap. I don't know if Ceria sells their OT as an individual item, but I figure any competent SE OT would do better than a thumb-sized Champ OT that rolls off steeply below 400 Hz.

[pdf64]

Perhaps the HT is excessive; maybe try dropping it back a bit to see if that helps, eg with an extra filter stage, as per the AC4 https://el34world.com/charts/Schemat...ox/Vox_ac4.pdf

[stephenhartley]

Thanks very much

I've used a Hammond 1760C which has 5000 and 8000 ohm taps - I'm using the 8000 ohm.

I've got a hefty 60s 10" Goodmans speaker.

The voltage measurements:

B+ 237
plate (pin7) 226
cathode (pin3) 5
pin 9 189

[stephenhartley]

The PT is a Danbury 1248DT - 190-0-190 @ 80mA

[Dave H]

I've used a Hammond 1760C which has 5000 and 8000 ohm taps - I'm using the 8000 ohm.

I've got a hefty 60s 10" Goodmans speaker.

The voltage measurements:

B+ 237
plate (pin7) 226
cathode (pin3) 5
pin 9 189

Try it on the 5k tap. With a low plate voltage like that it would be closer to centre biased on the 5k tap (like an AC4)

[stephenhartley]

Ok - I'll try the 5k tap - should I increase the cathode resistor?

[Dave H]

Ok - I'll try the 5k tap - should I increase the cathode resistor?

No, don't change the cathode resistor.

[stephenhartley]

5k tap seemed to make it worse - it cuts into a horrible buzz above 2 on vol now as though something is shorting

[Dave H]

How does it sound with the negative feedback resistor connected?