transistor voltage ratining Vs supply rails

[CBS240]

My agrees with Mark, the VAS duty is quite a lot for one transistor to handle alone. It is not a valve. Far better results can be obtained by using low Vce device with high Hfe for the current gain to reduce load on IPS, driving a common base stage with high Vce for voltage gain. Current gain device sees a constant Vce, greatly linearizing Ai. Also, common base can reduce the effects of the Early voltage.. Try to use low Cob device for CCS as this will be driven by the voltage gain collector.

 

[ctrlx]

using low Vce device with high Hfe for the current gain to reduce load on IPS, driving a common base stage with high Vce for voltage gain

not sure how to impliment this suggestion, i have attached the ltspice cct.

 

[Kay Pirinha]

It's a cascode, I'd say.
Best regards!

 

[ctrlx]

ok, so i have made the vas a cascode.....i can see that Q12 base-collector voltage is around 0.8v regardless of input level (ltspice sim)



some questions : what voltage to bias Q7 and why? is a current limiter Q13 needed?

 

[U230421]

Q12 is at 0.8 caused by Q13 when vas-current is =>6mA (iq13 is a current protection circuit for q12). And there is no voltage setpoint properly defined between Q3 and Q5, driving Q12 to max current (q13 in protection). Or it could have stuck to the minusrail.


Q7 at such level that Q12 stays out of ohmic region independend of load or Q7 output voltage swing. 2.2 is a bit low.

 

[JMFahey]

Back in the 70’s, BGW, Peavey, Bose, and others used to select 2N3773’s and 2N6259’s for 200 volts, to use them on up to +/-85V rails. They had better reliability than switching types that were actually rated 250V or more.

Been making Guitar amps (= abused amps ) since way back then and simply "there was no other option".
Used selected 2N3055 for decades (1969 to 2004/2006 when the fake situation got out of controkl) and 2N3773 until same happened about 2010/2012.
Added problem is that my amps have VERY long life (20/30 years being common) and for the last 10 years or so have been unable to get strong original types for servicing, so now I was forced to upgrade to MJ150xx

Some tried 2N3055’s but that didn’t go over so well. Using 3773’s at 200 V isn’t that much of a stretch - considering their rated SOA at 100 volts. I ran 3773’s up there for years, until you could get a 15024 for the same money. Back when it was common practice there was no MJ15024. There is no need to do it today.

Agree.
2N3055 can´t be used beyond +/-50V rails, and that with good selected ones; 2N3773 were my workhorse for decades until real ones disappeared and I was sold FAKE ones even by "reputable houses"

Current 2N3055 (I buy ST ones in original packaging straight from ARROW) are "good" but *just* meet published specs and are all Epitaxial, no Hometaxial ones available from any supplier any more, so there is a huge penalty on SOAR.
Fine to repair old Hi Fi stuff such as NAD amplifiers and similar 70´s, early 80´s "home" stuff, die miserably in Guitar Amp duty.

 

[29285]

2N3773 hometaxial from RCA were the workhorses of the seventies. They were the most rugged and the slowest as well. At the end of this decade the Hitachi lateral MOSFETs came into my focus: Far more rugged, and several magnitudes faster. In this aspect I disagree: There has been an alternative option

 

[rsavas]

At the end of this decade the Hitachi lateral MOSFETs came into my focus:

And now for new designs, you use what for laterals?

On the subject of 3055, anyone try out 2N3055AG?
2n3773 is still in production by OnSemi?

 

[29285]

And now for new designs, you use what for laterals?

On the subject of 3055, anyone try out 2N3055AG?
2n3773 is still in production by OnSemi?

I am out of the business of linear audio amps for decades. Just for fun I design a lateral MOSFET amp with original NOS 2SK135/J50.
For new designs I prefer TI class d-amp chips.

 

[rsavas]

Interesting, class D vs Lateral fets, I wonder if EB thinks the same. I tried TPA3100 & TPA3118, not the 3nd gen yet (TPA3251)
One big issue, I think, with D is matching the filters to the load Z. You change speaker Z, you change filters = PITA?

In one design I was looking at, ASTX, uses a green LED as the bias V across the 2T VAS cascode. iirc a Yamaha design grounded the base of the cascode. Bob Cordell shows a green led & a 1N4148 in his fig 3.12

I think you are better off using ksa992 or 2n5401 in the IPS. All 3 have different pin outs unless you cascode the bc556c lots of options.

If you can afford it, Bob Cordell's book covers these design techniques.