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12BH7 and questions about its operating conditions is AB or A1

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CaterpilarSK

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Joined 2018
#1
So I am trying to buil a obviously low power power amplifier with the 12BH7 as the output tube.

I cant seem to find much info on the internet on what the operating parameters should be for a single tube class AB or A1 power output configuration.

I dont see much info about what kind of plate load would be ideal and what kind of plate voltage would be idea to get the most of it while not too terribly damagin the tube. (And I have acess to custom wind stuff at home so a really speacial load impedance is not gonna cause any trouble).

I would like to use this in all kinds of things. Headphone amplifiers, laughable HIFI amps (you know in size and power), and a small guitar amp for my classmate and maybe a headphone ampliffier experiment aswell (maybe do a class A1 headphone amp, would be awesome). Designing a output stage isnt really a problem but I just cannot find the ideal plate load number or maybe I just wasnt looking hard enough. regardless if someone could help me out with this I would be thankfull 🙂


EDIT: Apparently I found my answer: 15-12.8Kohm impedance for that anode load (thats CT for AB) with a plate voltage of 250-260V (thats for the ECC99 so close enough id say).
EDIT2: And I also found a quite fragile output transformer from a battery radio that measures 10-11k impedance. I think I found the perfect match for a guitar amp..it even has the original enclosure and the original paper speaker in tack...great way to ressurect a otheriwse completely useless piece of trash into something nice
 
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#2

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#3
CaterpilarSK said:
So I am trying to buil a obviously low power power amplifier with the 12BH7 as the output tube.
Click to expand...

My question is: Why? Why use a small, expensive triode to build a power amp, when good Russian Military qualified 6P1P - EV tubes, perfect for small power amplifiers, could be had for under 2€ each? Even though JJ and EH still manufacture 12BH7s the NOS supply should ideally be saved for those who need them for existing equipment.

If you have a supply of good quality NOS 12BH7’s you could sell those for at least 15€ (and much higher depending on brand, etc.) per tube and buy all the 6P1P -EV tubes you need. They provide 12 watt of dissipation compared to the combined 7 watt of 12BH7 and are reported to be very good sounding tubes, especially triode connected.

My 2 cents. Best wishes with your project.
 
#4
Koonw said:
https://robrobinette.com/images/Guitar/Bassman_Micro/Bassman_Micro_LTP_Schematic.png
Bassman Micro
Use Hammond 125b OT
Other sites: hi-fi amp use TANGO U15-8 8kohm 15W
トランス結合　１２BH７A　パラ　PP　アンプ　（３） : まほろば　写真俳句
HIFIDIY- - Powered by Discuz!
http://www.single-ended.com/Lagarto/shishido/12BH7APP.png
Bassman Micro
Click to expand...

Why those extremely high grid stop resistors? Is that a typing error? Normally they are around 1k, here they are 1000 times higher. It should be very bad for treble reproduction due to miller cap getting into play.

Jan
 
#5
Bassman Micro LTP - YouTube
Listen to this, not very distorted but you hear a kind of "suspense" of sound? Many guitar amps has it higher than usual. But I'm no expert, someone can charm in.
Grid Resistors - Why Are They Used?

"The grid resistor on the preamp stages typically ranges from 0 to 68K, although very large values, such as 470K, are sometimes used in high-gain preamps to shape the frequency response and prevent "blocking" distortion in the preamp section under heavy overdrive conditions. The Miller capacitance of a typical 12AX7 is around 151pF, so the upper frequency response -3dB cutoff point of a stage using a 68K grid resistor is around 15.5kHz. The frequency response drops to around 2.2kHz if a 470k grid resistor is used. This "free" response rolloff can be used to tame the "buzziness" of high-gain preamp stages without having to add additional rolloff capacitors. Perhaps the most important grid resistor is the one that goes to the grid of the very first stage, right after the input jack. This resistor is the one that prevents oscillations and pickup of radio stations and other noise due to long or poorly-shielded cables. It is not usually a good idea to eliminate this resistor. Ideally, it should be soldered directly to the grid pins of the socket, with very short leads."
 
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#7
CaterpilarSK said:
So I am trying to buil a obviously low power power amplifier with the 12BH7 as the output tube.

I cant seem to find much info on the internet on what the operating parameters should be for a single tube class AB or A1 power output configuration.

I dont see much info about what kind of plate load would be ideal and what kind of plate voltage would be idea to get the most of it while not too terribly damagin the tube. (And I have acess to custom wind stuff at home so a really speacial load impedance is not gonna cause any trouble).

I would like to use this in all kinds of things. Headphone amplifiers, laughable HIFI amps (you know in size and power), and a small guitar amp for my classmate and maybe a headphone ampliffier experiment aswell (maybe do a class A1 headphone amp, would be awesome). Designing a output stage isnt really a problem but I just cannot find the ideal plate load number or maybe I just wasnt looking hard enough. regardless if someone could help me out with this I would be thankfull 🙂


EDIT: Apparently I found my answer: 15-12.8Kohm impedance for that anode load (thats CT for AB) with a plate voltage of 250-260V (thats for the ECC99 so close enough id say).
EDIT2: And I also found a quite fragile output transformer from a battery radio that measures 10-11k impedance. I think I found the perfect match for a guitar amp..it even has the original enclosure and the original paper speaker in tack...great way to ressurect a otheriwse completely useless piece of trash into something nice
Click to expand...
If it is a single ended amp you have no choice but use class A
 
#9
Francois G said:
My question is: Why? Why use a small, expensive triode to build a power amp, when good Russian Military qualified 6P1P - EV tubes, perfect for small power amplifiers, could be had for under 2€ each? Even though JJ and EH still manufacture 12BH7s the NOS supply should ideally be saved for those who need them for existing equipment.

If you have a supply of good quality NOS 12BH7’s you could sell those for at least 15€ (and much higher depending on brand, etc.) per tube and buy all the 6P1P -EV tubes you need. They provide 12 watt of dissipation compared to the combined 7 watt of 12BH7 and are reported to be very good sounding tubes, especially triode connected.

My 2 cents. Best wishes with your project.
Click to expand...

I don't think I asked for marketing advice. But I'll tell you the secret. Shuguang makes them and the are are suprisingly great quality tubes. I still end up cheaper than with a EL84 amp of any kind 🙂 12BH7 is a still manufactured tube I buy specifficaly newly manufactured tubes so I don't take away the good stuff from the people who need it.

It's my money my choice my decision 🙂
 
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#10
CaterpilarSK said:
The original transformer is a push pull one...it was likely operated in class B because it was a battery radio and it used 4 weird tubes in the output section. So I can build a class AB or A1 amp out of that 🙂
Click to expand...
I modded a little Magnavox SEP 6BQ5 amp into a 12BH7 SET a couple of years ago. Here's a link about the project:

Flea Power SET - Magnavox 12BH7 Mod

With a single 12BH7 per channel and the two triode sections in parallel, the stock Magnavox OT provided a load of 11.4k into 8 ohms, which is close to ideal.

Wouldn't PP would require a different load? Maybe something like 22k for a pair of tubes with their sections in parallel.

Or are you planning on using one tube per (stereo) channel and running the two triode sections separately in PP?

I've seen the new production 12BH7s by E-H and JJ but not Shuguang. Do they use a different number for the tube?

Please post a link to the Shuguang version, I found nothing when I did a search.
 
#11
Francois G said:
My question is: Why? Why use a small, expensive triode to build a power amp, when good Russian Military qualified 6P1P - EV tubes, perfect for small power amplifiers, could be had for under 2€ each? Even though JJ and EH still manufacture 12BH7s the NOS supply should ideally be saved for those who need them for existing equipment.

If you have a supply of good quality NOS 12BH7’s you could sell those for at least 15€ (and much higher depending on brand, etc.) per tube and buy all the 6P1P -EV tubes you need. They provide 12 watt of dissipation compared to the combined 7 watt of 12BH7 and are reported to be very good sounding tubes, especially triode connected.

My 2 cents. Best wishes with your project.
Click to expand...


6P1P-EV is really the way to go IMHO. They are really good quality for the price. and so cheap you can afford to get a decent stock of them for next to nothing.
 
#12
FlaCharlie said:
I modded a little Magnavox SEP 6BQ5 amp into a 12BH7 SET a couple of years ago. Here's a link about the project:

Flea Power SET - Magnavox 12BH7 Mod

With a single 12BH7 per channel and the two triode sections in parallel, the stock Magnavox OT provided a load of 11.4k into 8 ohms, which is close to ideal.

Wouldn't PP would require a different load? Maybe something like 22k for a pair of tubes with their sections in parallel.

Or are you planning on using one tube per (stereo) channel and running the two triode sections separately in PP?

I've seen the new production 12BH7s by E-H and JJ but not Shuguang. Do they use a different number for the tube?

Please post a link to the Shuguang version, I found nothing when I did a search.
Click to expand...

Yeah I playn to use the two triodes of a single tube in AB (no paralelling). The use the same tube and if you google "Shuguang 12BH7" you can buy some. I have not seen a datasheet from them tho. I assume any datasheet you can find online is accurate to it. Its the tubes printed as "CHINA 12BH7" but mine is blank actually.


For everone else in this thread. I kindly asked for ideal plate impedances and conditions for the 12BH7 not a discouragement to go with something else. At this rate I would get rid of tubes completely. Please stay on topic guys 😉 .
 
#13
We are trying to help you. No need to act defensively.

I googled, China 12BH7, Sino 12BH7 and “Shuguang 12BH7” and found almost nothing. There certainly are no large quantities of cheap 12BH7 that I could find. In ebay this is the top hit:
Shuguang 12BH7 hifi audio vacuum tube matched pair new (No print) | eBay

Of course $60 for a pair of 3.5 watt power dual triodes is not cheap, but you are free to spend your money anyway you like. The problem is that without markings on them it is not clear what the tubes actually are that you believe are “Shuguang 12BH7”. If they are true 12BH7 I would expect Shuguang to print that on the tubes. Perhaps you could post some photos of the tubes and links to places where these “Shuguang 12BH7” tubes could be bought.

So, without confirmation of what tubes you actually have, it will probably not be helpful to receive information from the forum that presumes a true 12BH7. I would suggest that you measure and draw your own performance curves to be sure.
 
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#14

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#15
I also encountered this listing (12BH7 不配对) but wondered what it really was. (And Fullmusic 12BH7s cost as much as the best of NOS).

Listed as 12BH7 5687 6N6 E182CC it is hard to tell what the tube characteristics actually might be. Granted those are similar but not identical tubes. Hence my suggestion that OP does some measurements to determine what he has.
 
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#18
Interesting. I checked my eBay orders and can't find the tube in the history. I am a 100% sure I bought a 12BH7 Shuguang back then for roughly not more than 15 dollars.

As far I know I do have the 12BH7 at least it looks like one. At the Slovakian tube supplier I can buy them for 17 euros which is still kinda acceptable. Same goes for the ECC99 which I would also like to try as a power amplifier.

JJ Electronic - 12BH7-A
JJ Electronic - ECC99

Here's the stuff I found from JJ.

But I can't seem to find any real documentation about the Chinese 12BH7 too. But I trust that it is the 12BH7
 
#19

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#20
Fortunately, I have enough in stock but I was curious about the Chinese version.

I found the unlabelled ones. Not sure I'd trust a tube with no markings whatsoever - unbranded and not even a tube type. I suspect these are bottom of the barrel since they don't even want to put their name on them. Pretty much everything else they sell is labelled, so why not these?

They are listed for $13 each and $24 a pair with free shipping. I saw other sellers listing them for $30 each.

Shuguang HiFi 12BH7(No print) | HiFiPART STORE

I tried some unlabelled Chinese 12AX7s years ago. The sections didn't match very well and I didn't care for them at all. I know that most buyers of inexpensive Chi-Fi amps replace the tubes immediately and report better results with just about any alternative.

If the OP doesn't have high expectations for his project they'd likely be fine. A cheap way to experiment with 12BH7s.

Not sure if they actually have any in stock, but the major dealer near me lists "generic" NOS for $21. I assume that means some brands are more. Prices have probably been driven up by the boutique dealers.
 
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