switching tubes: good or bad idea?

[szechuan]

using the circuit in the link below, my idea is to modify the existing circuit (re-biasing, etc) to accept different tubes, so that instead of using two different triodes (e88cc and gas7g) it will work with a dual triode. im trying to make a simple mono headphone amp that i can demonstrate to my classmates, as most of them have never heard a tube amp. just how easy would this be? what are some issues i'll probably have in attempting this? are there any dual triodes that could work in the existing circuit? sorry if i seem cheap or lazy, but i am on an extremely tight budget (though i already have a trafo for the heater, and some of the other passive components) and i'd like to accomplish as much as i can without sacrificing quality. also please do not suggest alternatives unless you are absolutely certain that you can save me a lot of trouble, as i chose this for its specific features as well as its simplicity and potentially low-ish cost

HeadWize - Project: A Single-Ended OTL Amplifier for Dynamic Headphones by Aren van Waarde

[DigitalJunkie]

The E88CC and 6AS7 are in fact dual triodes. That design uses one half per channel.

Perhaps you are looking for something with a half of an E88CC and a 6AS7 in the same bottle? (It's still going to use two tubes) And I'm not familiar with any tube that fits that description.

[Alain Poitras]

Originally Posted by szechuan

using the circuit in the link below, my idea is to modify the existing circuit (re-biasing, etc) to accept different tubes, so that instead of using two different triodes (e88cc and gas7g) it will work with a dual triode. im trying to make a simple mono headphone amp that i can demonstrate to my classmates, as most of them have never heard a tube amp. just how easy would this be? what are some issues i'll probably have in attempting this? are there any dual triodes that could work in the existing circuit? sorry if i seem cheap or lazy, but i am on an extremely tight budget (though i already have a trafo for the heater, and some of the other passive components) and i'd like to accomplish as much as i can without sacrificing quality. also please do not suggest alternatives unless you are absolutely certain that you can save me a lot of trouble, as i chose this for its specific features as well as its simplicity and potentially low-ish cost

HeadWize - Project: A Single-Ended OTL Amplifier for Dynamic Headphones by Aren van Waarde

The E88CC is a medium/high MU triode with is 33 amplification factor and is design to be operate under 130V between plate and cathode, it is a very good HIFI tube. It is equivalent to 6DJ8, 6922, ECC88 and CV2492 but all those tubes are very expensive (over 25$). A tube like 12AU7 is much cheaper (NOS about 12$ on eBay shipping included) but you will have to recalculate the bias and you will get less gain and more distorsion ... The 12AT7, 12AY7, 12BZ7 and 12AX7 have more gain but cost more than a 12AU7. It is also possible to use a 6N1P from Russia or China you can get for less than 11$ on eBay, shipping included ... They said they are the same as 6DJ8 but it is false, their curves are very different ...

But you can get a used but very good 6AS7G low MU power triode for between 15$ or 20$ on eBay shipping included, a good price. This tube is identical to the 6080 you can get for under 15$ on eBay ...

Power triodes are generally very expensive and the 6AS7 or 6080 are some of the rare ones you can get for a low price, it is very hard to find a cheaper alternative. They work at low voltage too and need a very high bias, that's why they are used in this direct coupling circuit ... They have very low gain, an amplification factor of only 2, that's why it is preferable to use a high gain preamp tube to drive them ...

I have the Spice models of those tubes and I will make some simulations with different drivers triodes and come back to you with the results !

I own a old Tektronix scope not working since many years and there is two 6080 RCA in it and twenty-two 6DJ8 Amperex, a total of 72 usefull tubes for audio ... I will borrow some from it to test my DIY circuits because I have an an extremely tight budget too and I try to use only the thousands of parts I already have in my "junkbox"... This scope is for me a real treasure ...

Cheers,

Alain.

[Globulator]

Originally Posted by Alain Poitras

I own a old Tektronix scope not working since many years and there is two 6080 RCA in it and twenty-two 6DJ8 Amperex, a total of 72 usefull tubes for audio ...

Me too - they are wonderful!
I use an Amperex bugleboy ECC88 from mine as the first stage (without feedback) in my SEP amp, it sounds most excellent

[kavermei]

Hi there

just my 2 cents, but if this is the first time you're building an amp I would strongly suggest to stick to existing designs... If you design the amp yourself you'll definitely need to do more trouble shooting which takes that much more experience to see through.

If you only use one channel, just connect all electrodes of the unused tube halves to GND.

IIRC, the russian 6N13S (in Cyrillic: 6H13C) is equivalent to the 6as7, it can be found pretty cheaply on Ebay and should be usable in the circuit you linked to.

To get the ECC88 cheaply you could try to find a PCC88 on Ebay, it was used a lot in european TVs. The heater requirements are a bit different though, the heater is rated for 300mA -- you could make a simple 300mA current source running off of 12VDC.

Have fun
Kenneth

[Alain Poitras]

Originally Posted by DigitalJunkie

The E88CC and 6AS7 are in fact dual triodes. That design uses one half per channel.

Perhaps you are looking for something with a half of an E88CC and a 6AS7 in the same bottle? (It's still going to use two tubes) And I'm not familiar with any tube that fits that description.

That's right ... I didn't notice he plan to build a monophonic amplifier ...

In that case, it can be possible to use a twin dissimilar triodes normally used for vertical deflection oscillator and amplifier in old black and white TV ... But is amplifier will have much less output power than with a 6AS7 / 6080 ... Type like miniature 9 pins 6CM7, 6DR7 and 6FD7 or octal 6DN7 and 6EM7/6EA7 can be used for that but I wonder if it is possible to find Spice models for them ... The old graphical loadlines method will then be have to be used ... They all cost 11$ or less on eBay with shipping, some NOS 6CM7 cost only 7$ ...

I will make my Spice simulations with the 6AS7 anyway to show the working principle of this circuit !

Alain.

[Alain Poitras]

Originally Posted by kavermei

Hi there

just my 2 cents, but if this is the first time you're building an amp I would strongly suggest to stick to existing designs... If you design the amp yourself you'll definitely need to do more trouble shooting which takes that much more experience to see through.

If you only use one channel, just connect all electrodes of the unused tube halves to GND.

IIRC, the russian 6N13S (in Cyrillic: 6H13C) is equivalent to the 6as7, it can be found pretty cheaply on Ebay and should be usable in the circuit you linked to.

To get the ECC88 cheaply you could try to find a PCC88 on Ebay, it was used a lot in european TVs. The heater requirements are a bit different though, the heater is rated for 300mA -- you could make a simple 300mA current source running off of 12VDC.

Have fun
Kenneth

He can also use the two 6AS7 triodes in parallel, same voltages but double current and more output power ... He will just have to use half the value of the autobias cathode resistor.

Alain.

[szechuan]

thanks for all the quick replies! and i have someone willing to donate whatever tube i need, and i offered to pay him back when i could. he has a fairly large stock, so as long as the tube isnt too exotic he should have it (one of the reasons i was hoping to find a good dual triode design). i realize it might be difficult to find two different triodes in a single device, i simply meant to get a dual triode that would be a good compromise between the e88cc and gas7. ive got no problem changing the bias either, as long as it can still operate at roughly the same V+, or at least wont need a negative rail for biasing. i'd like to minimize the needed power rails, i have a CT trafo for the heater already and thats really the only reason i can afford another for the main supply :P

[szechuan]

oh, and yes this is my first homemade tube amp, but ive got enough prior experience to avoid killing the tube, the other stuff, or myself anyway, biasing is FUN!!!!

[Alain Poitras]

Originally Posted by szechuan

thanks for all the quick replies! and i have someone willing to donate whatever tube i need, and i offered to pay him back when i could. he has a fairly large stock, so as long as the tube isnt too exotic he should have it (one of the reasons i was hoping to find a good dual triode design). i realize it might be difficult to find two different triodes in a single device, i simply meant to get a dual triode that would be a good compromise between the e88cc and gas7. ive got no problem changing the bias either, as long as it can still operate at roughly the same V+, or at least wont need a negative rail for biasing. i'd like to minimize the needed power rails, i have a CT trafo for the heater already and thats really the only reason i can afford another for the main supply :P

Like I write before, you can use a twin dissimilar triodes TV tube I give a list above, they all cost between 5$ and 10$ on eBay shipping included ...

But you have to know it is the power supply that will be the more expensive parts of your project because transformers are usually expensives ... Even if you use "back to back" low voltage transformers for heaters and high voltage.

Here's a Spice simulation of the original circuit I made :



I write all the voltages and currents on it. Also the power output and distortion for different input RMS voltages and load impedances.

As you can see, higher is the load impedance, better it is ... The author said it will work better with 600 ohms headphones and he is right. Usually, the load impedance have to be higher than the cathode autobias resistor to get the best sound, in this case, it should be higher than 3300 ohms ...

One way to raise the impedance of the load is to use a "speaker impedance matching tranformer", you can get one of 5 to 10 watts for about 6$ in some electronics stores ...

For example, a 100V line to 8 ohms have a ratio of 11,18 : 1 ... The impedance ratio is the square of 11,18 or 125 : 1 and with a 8 ohms speaker, the impedance "see" by the cathode follower stage will be 1000 ohms ...

Since there is no DC in the primary winding of this transformer, there will be no DC core saturation and a very small transformer can be used with very good results. Even some autotransformers or normal low voltage power transformers can be used if they have a suitable frequency response at low power ...

Look at this web page : http://www.freewebs.com/valvewizard/accf.html

They explain many things about cathode follower output stages !

But there is some easy and cheap way to ameliorate that circuit, by using a 3$ MOSFET current source instead of the 3300 ohms autobias resistor, all the AC current of the tube will then go into the load, no power loss and maximum power, also less distorsion ... I will be back next week with some ideas for you because I am also very interested by small low cost amplifier ...

Alain.

P.S. The 1 ohm resistor on the grid of the 6AS7 is just for the simulator, it is not there in the real cicuit ...