Chinese EL34 amp: your opinion please

[releone71]

I'm watching on the bay a 35W, EL34 powered, integrated amp which seems interesting. Point to point construction and attractive aluminum chassis. It's branded Rftlys model A1-EL34. There's also a more expensive variant with KT88. Never heard of the brand before. Have you ever had any experience with this brand/electronics? Your opinion about the specific model posted would be appreciated, as well as any suggestion regarding alternatives. Thanks for your attention!

 

[Rog Beltmann]

It looks like a nicely built amp. Good size transformers. I would try to get a service schematic in case it goes south. Looks like it uses fixed cathode bias rather than adjustable grid bias. Fixed bias is OK but grid bias is preferred.

 

[releone71]

@Rog Beltmann Thank you. Referring to the fixed bias, I can see four pots mounted over two small pcbs by the main tube sockets inside the case. Are those bias trimmers maybe?

 

[Rog Beltmann]

Yes. Those sound like the grid bias pots. If they are should only need adjustment when changing tubes. The owners manual should cover it if they are a user adjustment otherwise i'd leave em alone. My amp uses 4 tiny LED lamps. When the bias on each tube is adjusted correctly the lamp for that tube goes out. If your amp uses fixed bias there should be a fairly good size 230- 250 ohm resistor going from the cathode (pin 8 of the tube socket) to ground. If it uses grid bias pin 8 will go straight to ground.

 

[mcgjohn]

I have the RFTLYS amp. Very nice. Have done some upgrades to the coupling caps. another option in place of ebay is chinahifiaudio.com Have bought from them several times always with the amp as described. if you add copper caps to these amps they really come alive. I think the EL34 adn KT88 version are basically the same amp. If you re-adjust the bias you can run KT88s in the amp. Try some PSVANE T-IIs in the input stage and phase inverter. very nice amp. within its power range, can run with some much more expensive tube amps I have.

Here are a few pics of the parts upgrades. The amp is very well built. Very similar build quality as Primaluna....

 

[mcgjohn]

sorry forgot to add the link.

https://www.china-hifi-audio.com/en...r-c-40?zenid=0b74c4c277c86010f5b073af12478f04

 

[UncleBingo]

This is my favorite: :4 granules EL34B. Two granules 12AU7 one granule 12ax7

 

[releone71]

If it uses grid bias pin 8 will go straight to ground.

Thanks. I've seen a macro of those pcbs. The bias points are clearly printed over them. I guess you only need to insert the probes of your multimeter in the holes and read the value of the voltage.

 

[releone71]

if you add copper caps to these amps they really come alive. I think the EL34 adn KT88 version are basically the same amp. If you re-adjust the bias you can run KT88s in the amp.

Thanks for the infos. Yes, replacing the coupling caps is my idea too. Done with other Chinese stuff with great improvement. I know China Hi-fi Audio, have purchased other equipment in the past. Unfortunately they do not have the basic model in stock, just the blue tooth equipped. But I'm considering another site with the same prices...

 

[Rog Beltmann]

Thanks for the pics. Nice to see its mostly point to point wiring. Looks like a very well thought out design.

 

[FlaCharlie]

Looks like it uses fixed cathode bias rather than adjustable grid bias. Fixed bias is OK but grid bias is preferred.

I don't mean to nitpick but, just to clarify, because it IS confusing . . . "fixed" bias and "cathode bias" are two different things.

The term "fixed bias" actually refers to the type that is adjustable. Not sure who came up with the choice of terms. Perhaps they were thinking that you could "fix" the bias to whatever point you like.

Cathode bias is not adjustable. It is sometimes referred to as "auto bias".

As with most things, they each have their advantages and disadvantages.

Nice to see its mostly point to point wiring. Looks like a very well thought out design.

Even though there is a lot of point to point wiring, good luck finding a replacement if one of the boards has a problem. Not necessarily a deal breaker but it's something to consider. Of course, this holds true for pretty much any product that uses circuit boards. The unavailability of replacement boards, even for stuff made fairly recently, results in many products ending up in the scrap heap. All point to point is really the way to go.

Your opinion about the specific model posted would be appreciated, as well as any suggestion regarding alternatives.

This amp does look to be a considerable step up from the cheaper Chinese offerings.

That said, it's difficult to even find a schematic for most Chinese amps much less factory replacement parts. Maybe this company is different and has replacement boards for sale. You might want to ask before you buy. It would be a lot easier to have a replacement board sent to you than to have to return the amp to China for repair, assuming that the factory even has a service department, which is very unlikely.

At that price level, I'd consider spending a little more on something like one of the Dynaco clones. If the circuit board is damaged they are readily available. And if you're planning on doing some mods to the Chinese amp, you're getting even closer to the price of the Dynaco clones, which (I believe) can be ordered with "upgraded" parts.

I'd also suggest that you at least take a look at some vintage alternatives. Most are all point to point and, more importantly, vintage gear appreciates in value.

 

[releone71]

I'd also suggest that you at least take a look at some vintage alternatives. Most are all point to point and, more importantly, vintage gear appreciates in value.

Thanks for your point of view. To tell the truth, I'm fashinated by the Dynaco ST70. Moreover I own a pas 3x. Anyway most of them need to be restored. Would you please tell me more about those Dynaco clones?

 

[Rog Beltmann]

Fixed bias or cathode bias means just that... FIXED. Adjustable bias varies the negative voltage on the control grid to establish idle current. If you are confused check out a book on basic electronics. Other types of bias include grid leak bias and contact bias mainly used in voltage amplifier circuitry. Perhaps terminology has changed since my 1966 textbooks were written.

 

[mcgjohn]

nothing against a ST-70, I have had 4 of them. over the years. but this RFTLYS amp runs circles around the stock ST-70, and edges out the modified 70s I have.

Two other options you can do with this amp....

1. rebias the input socket to allow use of a 12AT7 in place of the 12AX7. You will lower the overall gain of the amp a bit, but its much more organic sounding.
2. replace the bias filter caps with some nice Nichon UPW types and bypass with a film cap.
3, the main power supply caps are nichcon so are good from the factory.

 

[FlaCharlie]

Thanks for your point of view. To tell the truth, I'm fashinated by the Dynaco ST70. Moreover I own a pas 3x. Anyway most of them need to be restored. Would you please tell me more about those Dynaco clones?

Original ST70s would need restoration, if they haven't already been restored. I have an old one that I need to rebuild someday. They are probably the most modded amp ever and there are many circuit board designs available but I know little, specifically, about them. Boards are available either blank or fully assembled.

@mcgjohn seems to have experience with both the Chinese amp and the Dynacos so I would defer to him on sonics. My point about the Dynaco types is that, even though they use circuit boards, the boards are available both in stock configuration and numerous modded versions. There are also Dynaco clone transformers, which many use for DIY projects.

I don't know much about the clones either except that there are a couple of companies making them and, I believe, someone also bought the Dynaco name and is selling them too. If you search on here, though, you'll find many people recommending the clones (some versions may not be actual clones of the original). A lot of people buy them in kit form and assemble them. Here's a couple of links:
http://www.tubes4hifi.com/bob.htm
https://www.dynakitparts.com/shop/st-70-kit-120-vac/

 

[FlaCharlie]

Perhaps terminology has changed since my 1966 textbooks were written.

Perhaps it has.

If you search on this, and other, tube amp forums you'll find that when people refer to fixed bias they are talking about amps which supply a negative voltage to the grid which, when used with output tubes, is adjustable with a pot.

 

[primosounds]

Fixed bias or cathode bias means just that... FIXED. Adjustable bias varies the negative voltage on the control grid to establish idle current. If you are confused check out a book on basic electronics.

Not to pile on but it really needs to be clarified that "fixed" bias always refers to an adjustable or sometimes "fixed" voltage that is imposed upon the input grid or G1 of the tube. It is usually negative but sometime even positive is used in some circuits.
Cathode bias via a resistor or a LED or battery or ?? is called Auto bias.

 

[Palustris]

Fixed bias or cathode bias means just that... FIXED. Adjustable bias varies the negative voltage on the control grid to establish idle current. If you are confused check out a book on basic electronics. Other types of bias include grid leak bias and contact bias mainly used in voltage amplifier circuitry. Perhaps terminology has changed since my 1966 textbooks were written.

There is absolutely no confusion here on FLCharlie's part and there has been no change in terminology. Fixed bias and self bias are terms that have been around for many decades and if you check the basic electronics books yourself you will discover this. Fixed bias means a negative voltage is applied to the control grid and self bias means that a resistor in the cathode provides the bias.

Classic text: Frederick Terman Electronic and Radio Engineering, 1955; Chapter 8, page 253, Figure 8-1 "fixed bias" and "self bias"

 

[releone71]

nothing against a ST-70, I have had 4 of them. over the years. but this RFTLYS amp runs circles around the stock ST-70, and edges out the modified 70s I have.

It seems A1 and A2 models are out of stock in China Hifi Audio's online store. It's a pity because the latest release of the A2 was updated with SCR film foil caps. The current A2 has been updated with the Bluetooth receiver, but…...is it wise to put a receiver together with tubes? Moreover it seems, as far as I can see by the pictures, the power supply stage has been downgraded in order to make room for the BT receiver. And I can't see the SCR caps anymore. Old vs. new A2:

 

[Lavane]

I built one of the Dynaclones myself. The big deciding factor for me was how much info was available and pmenty of replacement parts. Plus you can go mild or wild with the build.