• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

EL84 Amp - Baby Huey

No, it is a video pentode with sync separation/amplificaton triode.

Curves look ok.

I cranked a pair up to 12W plate dissipation/tube before I got one to glow. It broke down above 400V B+ :eek: pumping out close to 10W.

I know it is most commonly described as video pentode. I was talking about the datasheet that I have now attached.
Sounds nice they are willing to take some abuse!
 

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Wow! I hadn't seen that.

Class AB1 looks pretty close to what I was running. IIRC, I had a 8K5:4 transformer with a 8ohm speaker on the 4 ohm tap for 17K running 325-350V B+ for most of my testing. I tried several 6DX8s and 6F4Ps. They all seemed to be nearly identical in the abuse they would take.
 
Is it possible to build the baby huey with the following components :

Spark output iron with 4,6Kohm impedance, and UL and primare winding for catode load.
I can have 8 pieces of 6V6 tubes from a find, can they be used?

This will be my first tube amp build. I have read a lot about tube amp circuits, but I still have a lot to learn.
I understand from some of the first post,s that the 803s input tube can not drive a 6V6 and probably even less 2x2 6V6 tubes in each channel. But why is that? is it gain or load or ...?
i would have prefered that simple all tube two gain stage circuit.
 
Anything can work, the question is whether can it produce fantastic results. Since it is your 1st build, just use whatever you have & see if they work. Tubes are fantastically tolerant to brutality.
BTW, 803 is a bigger power tube than a 6V6 both physically & electronic specifications. are you sure you wanna use 803 to drive 6V6? I bet a pair of 803 can produce as much or more power than you 8 pieces of 6V6.
 
I think Hylle was referring to the ECC803S (12AX7) not a 803 output tube.

The problem with using 6V6 output tubes is that the maximum grid 1 resistance for fixed bias is 100K. With cathode bias you can use 470K.
The basic Baby Huey (the circuit back on page 1 or 2) will therefore work OK just substituting 6V6 in place of the EL84. the ECC803S or any 12AX7 equivalent (6N2P for example) should drive them just fine.

The next problem is that the 4,6 KOhm output transformer is a little low in it's primary impedance for 6V6. You could try hooking an 8 Ohm speaker to the 4 Ohm output winding, that will reflect 9,2KOhms at the primary, which is good for 6V6.

Hope this answers you questions.

Cheers,
Ian
 
No my mistake, I should have been more specific concerning the ECC803S tube.

I plan to use two parallel highside and lowside tubes, meaning 4 pc. 6V6 output tubes in each channel, that should fit the transformer impedance better?
Can they still be driven by on ECC803S /12AX7 tube?

If this will work, why was a source follower used in the 6V6 Baby Huey in earlier posts?

gingertube, you make me nervose, when you start you reply with "The problem...."
:), thanks guys for all you help!
 
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If this will work, why was a source follower used in the 6V6 Baby Huey in earlier posts?

There is also an EL84 version of the BH with mosfet followers; the mosfet followers provide the needed current to the output tubes (to drive the miller capacitance), relieving the input tube of the need to provide lots of current when they are high mu, low gm tubes like the 12AX7.

Do your output transformers have 4 ohm speaker taps?
 
Ian,
What do you think of instead of 12AX7/ECC83/ECC803, a cascode differential with a low capacitance JFET at the bottom & a 6DJ8/6922/ E88CC/ECC88 at the top. Should this give a better drive to get over the miller's capacitance of the output tube? This also opens up choices of FET since the voltage rating is lower, AFAIK, this will be easier to implement then a MOSFET source follower...
I don't think it will complicate your streamlined circuit any more than adding a source follower MOSFET.
 
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Ian,
What do you think of instead of 12AX7/ECC83/ECC803, a cascode differential with a low capacitance JFET at the bottom & a 6DJ8/6922/ E88CC/ECC88 at the top. Should this give a better drive to get over the miller's capacitance of the output tube? This also opens up choices of FET since the voltage rating is lower, AFAIK, this will be easier to implement then a MOSFET source follower...
I don't think it will complicate your streamlined circuit any more than adding a source follower MOSFET.

No, a cascode has very high output impedance
 
Hum, and transformer grounding??

Hello all,

So I get mounting the power transformer on the chassis and only have one grounding point on the chassis to reduce hum. My transformer also have an earthing lug on one of the bolts that clamps the laminations (top of transformer).

So, question..

Go with the standard option of just mounting the Xformer on the chassis and making sure there is good conductively between the base and the chassis (free of resin etc), or isolating the xformer from the chassis and connecting the earth lug back to the single earth point on the chassis via a substantial cable?

Thanks in advance ;-)
 
Hylle,
All of the 6V6 versions I built and have posted here were fixed bias. That meant a maximum value of 100K for Rg1 on the output tubes. To accommodate that I used the source follower.
If you are going to use push pull parallel 6V6, then the Rg1 are in parallel and have a maximum value of 50K for fixed bias and 220K for cathode bias. Using the source followers becomes essential to buffer the front end.
Cheers,
Ian
 
Audio_Idiot and others with questions above,
The requirement for the BH front end is low current (approx 0,6mA per side of the diffamp) and high rp. The high rp is required so that the feedback is not divided down too much by the internal impedance (rp) of the tube.
The low current is required to keep the anode load resistors high whilst maintaining resonable voltages at the anodes of the diffamp. Be aware that 12AX7 can run into grid current problems at low anode voltages.

Cascodes have high rp and would therefore be suitable. Choosing something which will run at low currents is the next trick. A JFET triode cascode diff amp would work well and in fact that would be a good solution for those who wanted to build a 2 tube per channel 6BM8 Triode/Pentode based BH.
6DJ8 have a reputation of not being great at low currents but I have seen them used at low current. If doing a JFET triode cascode diff amp I would probably choose BF245 plus 12AT7 but try the JFET/6DJ8 cascode by all means. There is also another option - The gain of a cascode is proportional to the gm of the lower device x the upper device load resistor. Bipolar transistors are the kings of gm. Perhaps try a matched pair of bipolar transistors rather than JFETs. I would need to look up the part number but 2N5551 and 2N5401 (can never remeber which is the npn and which is the pnp) are both available as matched pairs in a surface mount chip. For the bottom of the cascode you need npn.

I've been using a 6V6 / 6SL7 BH for many months now.
MY EL84 BH monoblocks are STILL sitting on the workshop bench to have their front ends re-done using 6BR7 (EF86-ish) pentodes for the diffamp.
Malcolmfraser,
Just ground the transformer to the case. To keep noise low you should have a single connection for the 0V (signal ground) to chassis. Keep this connection away (opposite side of chassis if you can manage it) to the safety ground chassis connection. If the power tranny has a shield then connect that to the safety ground as well, rather than the signal ground.

Hope I've caught up with all the questions.

Cheers,
Ian
 
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