300b amp speaker driving problems

[Gideon300b]

Hi,

I hope some of you can help me. I am building a 300b stereo power amp with Phil Riley of Bluebell audio. Whe I finish the project I'll post some piccys. Doing it this way is very stimulating and challenging - however have hit a oroblem which is foxing us, (well this evening anyway.

I have got the amp up and running with a paitr of cheapo 10 ohm surround speakers. When I run it up against my IPL STL4's (measuring 6.2 ohms) nothing comes out the other end - well the volume is tiny.

Any clues?

Thanks all.

Gideon

[NickG]

Does it work ok with the surround speakers?

Have you another amplifier to connect to the IPL's to make sure there is nothing wrong with them and the connection to them?

[andrew ivimey]

Gideon, you haven't given us much to go on.

1. Philip's 300b is a version of the WE91 design. It is marvellous.

2. Your IPL speakers work fine do they with another amp.

3. What is the sopund like coming out of the 'surround speakers' - it really doesn't matter about the impedance, if these are small cheap and nasties the amp should blow them away.

4. Is it the same on both channels??? If you are using one power supply then teh prob is somewhere there. If you have monoblocked, you have made the same mistake twice!

5. Visually check and check again that the circuit is right.

I have had wonderful luck in building amplifiers but when I built a WE91 variant, for some inexplicable reason (premature alzheimers, too much red wine!) I wired the 300b cathode resistors and cap wrongly - really simple mistake - on both 300bs. It didn't sound very good but like with most valve circuits; they are forgiving and I got it right haing beaten myself about the head for foolishness.

4. Has Philip given you any pointers to correct voltages? Measure them!

Load the speaker outputs with power resistors and get your multimeer out. Are you using one mains transformer or monoblocking with two? The HT should read around 400volts with everything on circuit.

I have tried 300volts and 500 volts with those huge Czech 300bs. There is a power resistor after the second PSU cap??? How hot does it get? It should get warm to hot depending on type and how you (if you) have heatsunk it.

That's a starter, as I say if you are using one PSU, the problem prob. lies there. If you can't see the mistake you have made you must measure voltages and get realistic voltages where they are supposed to be. Philip should be able to help but for courtesy's sake let the phone bill be your's!

Good luck - it is worth persevering.

[Gideon300b]

Hi Andrew and thanks

Oh Im not giving up - its good fun, sometimes frustrating and Phil is a gem.

Answer to your queations were almost all yes

Its a single tranny feeding a pair of 300bs following the 91 circuit. The IpLS are fine with my other amp. The surriunfd sounds are fuine with the 300b in that they go as well as you can expect.

I was just seeing if anyone has any clues as I am now waiting for Phil to send down some more bits (resistors). Waiting for him to send is the only frustrating bit.

Gideon

[andrew ivimey]

Okay Gideon, well how vital are the er, missing resistors?

Take a break from the amp, come back to it tomorrow, when refreshed. Check the circuit schematic again. It is so easy to make a simple mistake that affects things globally.

If those resistors are not vital, ( i.e. just upgrading what is basically right spec. and already there,) then there has to be a PSU prob. if both channels are fed by the PSU and they both sound the same ... or ... you have made the same mistake twice.

(If those resistors are missing - switch off, leave well alone and wait for postmanpat. this design is 'slim' with no room for cutting any more corners)

This is easily possibly, as I said before, have a think and check the circuit, have another think.

Now what's left ????

It is a pentode driver which is a little more complicated that the usual triode. Check the components around those grids!

There is a coupling cap, from the anode to the 300b grid? I think.
Is that right?

Have you AC heater on the 300bs ? So, where are the cathode cap and 800ohm resistor?

Your output transformers ... that's a thought ... check primary between anode 300b and HT (B+)

next?

[Gideon300b]

Thanks Andrew

Ill discus some of these ideas with Phil.

Ive chacked the circuit and layouts thoroughly but hadn't thought of testing the primary Op tx against the HT.

I'll wait a couple of days for the bits to arrive. What I cant still get my head around is how it works with one set of speakers and not another. All good fun.

[simon2a3]

Originally Posted by andrew ivimey

Your output transformers ... that's a thought ... check primary between anode 300b and HT (B+)

Hi Andrew, Gideon300b.

I've just completed my Philip Ramsey WE91 monos so I'm interested in this. I get 17V between 300b anode and B+, 400V between 300b anode and ground. Does this sound about right?

Simon.

[Gideon300b]

Hi Simon

From memory my voltages were higher than 400v on the anode. What sort of speakers are you driving wiht your monoblocks. I am guessing that your circuits is identical except for the power supply?
Thanks

[Gideon300b]

Hi Simon

From memory my voltages were higher than 400v on the anode. What sort of speakers are you driving wiht your monoblocks. I am guessing that your circuits is identical except for the power supply?
Thanks

[Richard]

Originally Posted by Gideon300b

Hi,

I hope some of you can help me. I am building a 300b stereo power amp with Phil Riley of Bluebell audio. Whe I finish the project I'll post some piccys. Doing it this way is very stimulating and challenging - however have hit a oroblem which is foxing us, (well this evening anyway.

I have got the amp up and running with a paitr of cheapo 10 ohm surround speakers. When I run it up against my IPL STL4's (measuring 6.2 ohms) nothing comes out the other end - well the volume is tiny.

Any clues?

Thanks all.

Gideon

Hi Gideon,

The first thing to do after hooking up and powering a new project is to check the circuit voltages. these will show you where the problem lies.

Rich