Converting EL38 amp to 300b Amp

[MistyBlue]

Misty Blue, I got there first to prize the protective layer away.

Oooooo.....brushed........nice!

[Paul Barker]

I think I’ll leave it alone. I might pit the valve bases in two rails, underneath and let the valves pass through to the screwed bases out of sight. The only downside is the 300b gold writing wouldn’t show. But if I level the 45’s and 300b’s just at the top of the base the benefit of a smooth top plate would be more rewarding than the hash up I might make of the screws into the bases.

If I level the 6J5 as pictured, to change those valves, remove other vlves and lift top plate off.




Oops the GEC valve is a 6C5. As they are slightly inferior I’ll find a 6J5 to swop it out. Although the differences between wont affect anything in this amp, so I have the other option of finding another 6C5 inferior meant slightly higher iR. Sound probably not the issue.

So the two rails with the valve bases could mount side to side to the side panels. Cover the side panels after with perhaps Bamboo cutting board material.

somewhere I have quit a few Danby smoothing chokes which were nominally 10h but I always measured 8h. I think they were more than 50mA, possibly 100mA. As the 45’s are running at 40mA and the 6J5 at 5mA those will do the first level smoothing at the 45’s. The 150h hammonds 8 mA can still go at the first stage.

So those chokes could go inside and the R cores back on top. Next I have to experiment with a low level case for the R cores. I have seen a cheap glass panel lantern at poundstretcher but it may not contain the R cores. So I’ll go with one R cores to experiment with repurpose options. I dont mind that the lanterns are glass sided. Having site of whats inside is no bad thing.

[Paul Barker]

Then there are the new requirements for the 8 ohm speakers.

Well I have some of these, which sound good.

[Paul Barker]

oh yes transconductance low

On that thought, I would say that the 45 is hardly a valve I would pick for what is basically a cathode follower. gm is your friend.

You’re right of course.

Well Ive never really heard a good 6550 amp, and even recently I tried it but the EL38 was the best sounding se triode, But I was fighting the top cap issue. But if I allow the top caps the EL38 or the 12e1 both have very high transconductance.

But I have found an alternative in parallel 5687 at the required operating point in parallel s is 13 (6.5 each half) current flow combined 30mA. I have just enough 5687’s.

Whaat do you say?

[Nick]

Well, its old news, but I remember that high gm pentodes did a very good job of driving 300b's.

I would think one 5687 would pass enough current without needing 30ma. Works fine on the 300b's I built. Its just a matter of driving the miller of the 300b at the higest frequency and swing that you are likely to need. Its unlikely that you will be swinging 140v P/P at 40kHz. But even if you were, that's about 2.6ma (RMS) current into the grid capacitance. I would have said anything with a output impedance less than 5k would be fine.

I used to use a resistance loaded, cap coupled 5686 (bipassed cathode) directly coupled to a choke (or interstage primary) loaded second 5687 worked well, play with the cap on the second 5687 cathode to work with the load capacitance to peak the bass response to make the most of the interstage.

[simon]

Well, its old news, but I remember that high gm pentodes did a very good job of driving 300b's.

If I ever did 300B again I think I'd have to try a D3a

[Nick]

Getting enough voltage swing is a problem with the d3a. Or at least I think that was the case.

[Paul Barker]

I just remembered I wanted to use octal or bigger bases for dexterity reasons.

6BX7 : 7.5 mA/v current 40mA will swing 150v peak

Or nail the output impedance with the 12e1 14mA/v and do a safety cage thing. I have more than sufficient 12e1’s . Only possible issue is I have never heard them.

If operated at 100mA to lift it out of its poor distortion at lower current it’s 2.8 thd. Each monoblock power supply is 250 watt so there is no shortage of power to give 100mA to the driver stage. Ive shown the voltage swing only at 100v peak to see what the distortion would be in the real world voltage swing not the excessive voltage swing. But there is plenty of headroom.

On its own it will experience grid current at -1v, but it could be kept away from that.

Same voltage driver element would drive a GM70 output stage, so when it’s day arrives the GM 70 only requires its HT supply and the output transformers. Not a small matter. But I think all things considered I’ll stop at the GM70 in this lifetime I have realistically left. And only that if I can improved my blood pressure, which is going in the right direction but has a way to go to push me past the 10 year mark, from statistical life expectancy for a post stroke or heart attack longevity Mean.

[Paul Barker]

Well, its old news, but I remember that high gm pentodes did a very good job of driving 300b's.

If I ever did 300B again I think I'd have to try a D3a

OK Simon in my srpp situation:

Drop the b+ to 360v

Anode Volts 180 due to g2 max 180v

Bias volts -1.62
Current 25 mA max for 4.5 watts
Bias resistor 65 ohm
Mu 75
ri 1.8 kohm
Transconductance 42mA/V

Voltage swing 90v peak just comes out enough with 300b bias of 84.

It just happens to work from all angles. Only need a few volts input gain perhaps the 45 expected gain of 3.

Now all my punched valve holes are trashed LOL i could punch a mounting hole in fine mesh and fill the 3.5 CM holes on the pretence it was designed for valve cooling. For the row of four at the back, and turn thr outer front holes to 3.5 cm for the two 45’s.

[Paul Barker]

Well, its old news, but I remember that high gm pentodes did a very good job of driving 300b's.

If I ever did 300B again I think I'd have to try a D3a

OK Simon in my srpp situation:

Drop the b+ to 360v

Anode Volts 180 due to g2 max 180v

Bias volts -1.62
Current 25 mA max for 4.5 watts
Bias resistor 65 ohm
Mu 75
ri 1.8 kohm
Transconductance 42mA/V

Voltage swing 90v peak just comes out enough with 300b bias of 84.

It just happens to work from all angles. Only need a few volts input gain perhaps the 45 expected gain of 3.

Now all my punched valve holes are trashed LOL i could punch a mounting hole in fine mesh and fill the 3.5 CM holes on the pretence it was designed for valve cooling. For the row of four at the back, and turn thr outer front holes to 3.5 cm for the two 45’s.

No top caps.

[simon]

Getting enough voltage swing is a problem with the d3a. Or at least I think that was the case.

It's so long since I've used 300B I had to look at the datasheet again. For the higher HT op of 350V the grid voltage is -74V. I seem to remember getting a gain of around 70 in triode, so close enough?

Paul's done proper calcs though.

[Nick]

Yes, it can be done with 20ma, and triode mode and a 10k load with 350v B+. If we are happy with close enough, then yep.

I keep forgetting we are in "near enough, that will do" land.

[Paul Barker]

I dont call 90 v peak close enough land it past the required voltage of 84v, if it were under 84 v it wouldnt be close enough for me , it wouldnt be built. It is beyond the requirement so it gets built.

Iam not in near enough that will do land.

[simon]

I think that was a reply to me rather than you Paul.

Okay so it might need a bit more voltage input that 1V, but everything's a compromise.

I haven't looked at any design so there may be more going on that I haven't considered.

[Paul Barker]

Yes it needs an iput stage, not much voltage gain so I went for 45. I could have looked for a british small signal dht from the very old days but 45’s proliferate and do the job, 10y is much loved in a preamp but who needs voltage gain of 7.5?

My quiescent current for the d3a is 25mA