What do you think about this amp?

[Davebassman]

I've been wanting to build a tube amp for some time. I found this project some years ago. It's push pull tube with EL34 and 30watt of power. I draw the layout in kicad. But is it worth it? What do you think?

Thanks.

[Chuck H]

Who designed this amp and what draws you to it?

It has aspects of both a guitar amp and a hi fi amp. What do you intend to use it for?

[Davebassman]

Who designed this amp and what draws you to it?


It has aspects of both a guitar amp and a hi fi amp. What do you intend to use it for?

It's from an old electronics magazine. I have some photos of the layout so I re-draw in kicad. Because I have the tubes, and some material to build it. It seems that it's not complicated at all.

It's a HIFI amp.

[Chuck H]

Well it does have more in common with a hi fi design. Though some circuits and values seem odd for that. I don't know enough about hi fi design to comment though. Except to say that the global feedback loop values will need to be tweaked unless you'll be using the exact same output transformer that applies to that schematic. Which seems unlikely. That and, just my own personal position, I would want to build two of something for stereo.

[Mick Bailey]

It will make a sound, if that's all you want. I wouldn't bother with this myself for guitar. If you already have the transformers I would redesign it around the Dr. Z Route 66 schematic which has a similar topology, but with EL34 outputs so you can use your tubes and transformer.

[Chuck H]

Not sure what you're planning to do about a power supply either as it's not included in the schematic. You could also build something venerable and known like a Mullard 520. Same EF86 and EL34 tubes but uses a 12ax7 for the phase inverter. Since 12ax7's aren't hard to come by I wouldn't think that tube is a deal breaker. Plus you'll be able to find a lot more info on line for something like the Mullard 520. Discussions about builds on forums and such. With the above circuit all you can get is probabilities and opinions because no one knows the design.

[Chuck H]

I don't see any reason you couldn't skip the tube rectifier and choke feed for the Mullard 520 power supply. Using modern filter capacitors available/affordable in higher values I think a diode rectifier and a Pi filter arrangement should work fine and cost a lot less.

[Davebassman]

Not sure what you're planning to do about a power supply either as it's not included in the schematic. You could also build something venerable and known like a Mullard 520. Same EF86 and EL34 tubes but uses a 12ax7 for the phase inverter. Since 12ax7's aren't hard to come by I wouldn't thing that tube is a deal breaker. Plus you'll be able to find a lot more info on line for something like the Mullard 520. Discussions about builds on forums and such. With the above circuit all you can get is probabilities and opinions because no one knows the design.

Wow, It's possible that the project that I upload it's based on this mullard? it's very similar. The MK4000 uses ECC81 for the phase inverter. The power supply schematic is on the PDF that I uploaded.

[Davebassman]

I don't see any reason you couldn't skip the tube rectifier and choke feed for the power supply. Using modern filter capacitors available/affordable in higher values I think a diode rectifier and a Pi filter arrangement should work fine and cost a lot less.

I think the same. This is the schematic for the power supply.

[Helmholtz]

Quality and stability of such design depend on the OT. Means parameters like primary inductance, self-capacitance and leakage inductance.

[Chuck H]

Wow, It's possible that the project that I upload it's based on this mullard? it's very similar. The MK4000 uses ECC81 for the phase inverter. The power supply schematic is on the PDF that I uploaded.

Probably based on the Mullard but shooting for more power (likely at the sacrifice of greater distortion). Odd things about the MK4000 that I can see are a comparably small amount of feedback considering loop gain. The 1n bypass cap on the 100R V1 tail/NFB shunt resistor defeats HF from the feedback loop. Seemingly high values for the phase inverter grid loads and power tube grid loads. In fact I think the power tube grid load max value is 500k even for cathode bias. Not to mention that some think the grid load value should be kept lower than typical for modern EL34 tubes. So that seemed odd.

[Helmholtz]

Odd things about the MK4000 that I can see are a comparably small amount of feedback considering loop gain.

I assume the NFB ratio (open loop gain divided by closed loop gain) is large compared to a typical guitar amp circuit, as the open loop gain might be huge due to the high gain of the (additional) pentode in the loop.

The NFB voltage divider only sets the closed loop gain. It doesn't tell about the actual amount of NFB.

The larger the NFB ratio the more critical the parasitics of the OT.

[Mick Bailey]

If you're thinking of a hi-fi amp you'll need two of everything except the PSU for stereo operation. Plus, depending on your input source you may also need a preamp. There's also no RIAA correction for Vinyl.

[Davebassman]

If you're thinking of a hi-fi amp you'll need two of everything except the PSU for stereo operation. Plus, depending on your input source you may also need a preamp. There's also no RIAA correction for Vinyl.

Yes, I know. It seems that it is originally designed to plug a CD or nowadays a streaming DAC. Anyway I have a turntable preamp.

[Chuck H]

Anyway I have a turntable preamp.

With a mono output?

I'm planning a couple of HI FI builds at some point. But not before I can make FOUR of them. That's right, four. Well actually two with stereo channels, so only one PS per unit. But I've always wanted to make one for my brother and I've wanted to make one for myself. It's on the back burner but I'll get to it. And I'm not going with big watts either. I just don't need them anymore so it only increases the expense. Nope. I'll be doing ten watts per channel. My point is...

I'll sacrifice watts, but why sacrifice STEREO!?! Are you building a tube HI FI amp for an older mono output turntable with a built in preamp? Just seems like a long way to go. If the record player is sentimental for some reason, I can get behind that for sure. But if it's that sort of thing you really don't need a fine tuned tube mono block to acceptably recreate whatever cones out of it. A single mono block just seems to have really limited application. Especially since everything is (at least) in stereo. So other than gear with mono outputs (and likely lower quality signal than merits a tube mono block build) all you can do is listen to the R channel or the L channel. Well, some have a bridge/mono option but I'm not sure anymore what might include that.

JM2C