dumble feeddback ?

[Marc]

I was wondering if a dumble style feed loop at the preamp would help to control feedback? I was thinking the 10M resistor and 0.01 cap in series from the plate to the input on the preamp section would tame the tendency of at least that section from feeding back.
Is that even worth pursuing or is most feedback generated in the power section?

I don't have a good enough understanding of how feedback is generated in a harp amp to describe in latman's terms what is happening- at least not the right words to explain to a harp player why sticking your harp into the speaker causes feedback....any simple explaination would be geatly appreciated.

The THD style lamp expander works well on the OT to supress feedback but tends to suck output. Has anyne tried a similar circuit in the preamp section?


Marc

[MWJB]

Not familiar with the circuit you mention.

Feedback is generated/exacerbated in preamp & power amp sections, as well as the fact that different speakers & brands of power tube have wildly differing thresholds to feedback. Really you want an approach that limits feedback, to a degree, all the way through the amp. A few % here and there add up to make a tangible advantage, areas you might look at are preamp plate voltages, attenuating high end, reducing gain, plate current, feedback resistant power tubes & speakers. Focusing on extreme measures at localised points in the amp can lead to robbing Peter at that stage, only to to pay Paul at a later stage in the amp... and for every action theres a reaction...reduce high end and you typically get worse fidelity, reduce gain too much and you lose punch, dynamics & low end. You might find a speaker that is resistant to feedback but is too dark/innefficient to be useful. As far as tubes and speakers go, ear testing is key, but quickly gets very expensive.

I tend to work to a "just enough" policy with regard to preamp voltage and plate current, dictated by the player's preferences (some guys just have to have so many mA). An amp that one guy finds clean as a whistle can turn to mud in someone else's hands. So a degree of user adjustability on these fronts can help you cover more than one base.

Bear in mind that some amps that have relatively high gain (from a harp player's point of view) are still very desirable, e.g brown Concerts, Bandmasters, Super Reverbs & Twins. Some great guitar amps just need a little fine tuning to be great stage harp amps...some others are so labour intensive you may as well start from scratch/buy something else.

At the end of the day the individual player will want a degree of highs and fidelity that THEY need to be happy & get accross...some will want more or less than others (just like some guitarists might love a 5C5 Pro and others will always prefer a BFSR).

It's not your harp that causes the feedback per se, it's the method used to amplify it. Imagine you take an acoustic guitar, instead of fitting a magnetic pick up you stick an old Hi-Z mic over the sound hole and plug it into an amp Do you think you'd be heard with a band before feedback set in? A harp with magnetic pick ups would solve many of the "problems" associated with amplified harp. Bulk & cost would be prohibitive. If one was built, would it sound better than the "Heath Robbo" techniques harp players have used since the '50s to record classic sounds? Can't help wondering...

[Bruce / Mission Amps]

I'm not sure why other then he doesn't want my name on his kits, but as of March 2007, I noticed Weber has taken my name of the small KT66 5F2H harp amp I designed.
It is based on an old Premier Twin 8 of mine.

Regardless, it has a Dumbly local feed back in it at the first stage.
Not to cut feedback (it is broad banded local FB) but to control some of the hot gain and smooth out the frequency response a bit.
I just liked the way it sounded better with it and that modified tone stack.

https://taweber.powweb.com/store/5f2h_schem.jpg

[Steve A.]

I'm not sure why other then he doesn't want my name on his kits, but as of March 2007, I noticed Weber has taken my name of the small KT66 5F2H harp amp I designed.
It is based on an old Premier Twin 8 of mine.

Regardless, it has a Dumbly local feed back in it at the first stage.
Not to cut feedback (it is broad banded local FB) but to control some of the hot gain and smooth out the frequency response a bit.
I just liked the way it sounded better with it and that modified tone stack.

https://taweber.powweb.com/store/5f2h_schem.jpg

So they used to call this amp the "Bruce"??? The circuit looks really slick. How does it work for guitar? Does WeberVST sell just the transformers and choke, or just the whole kit? And with the KT66, I would imagine that it puts out a little more oomph than a 6L6...

Steve Ahola

[Bruce / Mission Amps]

So they used to call this amp the "Bruce"??? The circuit looks really slick. How does it work for guitar? Does WeberVST sell just the transformers and choke, or just the whole kit? And with the KT66, I would imagine that it puts out a little more oomph than a 6L6...

Steve Ahola

I don't know what they sell ... I don't do business with them anymore.

Bruce amp... No , I always called it the 5F2AH or 5F2H amp... way before I drew up the schematic for Weber.
The schematic and layouts used to say designed by Bruce Collins on the Weber website.

It's a version of an old Premier Twin 8 amp of mine where I modded it a bit for a Harp player friend of mine... when it just sounded so damn good using a choke input KT66, bigger trannys and some tone mods, I started building the thing on separate Soulkicker Jr. and or Starlite chassis with a single 10" speaker for some other local players.
Yes it can be tweaked a little for guitar too.
A few years ago Weber had been asking me to design a few amps for them, using my name to help with sales and I mentioned the 5F2H and the 5E3X2.
They did both of them.
But some things happened between us after that fiasco that pissed me and Weber off and now, (like I've heard they have done to other builders), they have me banned from the Weber Website and removed most all mention of me and Mission Amps.
Ha ha ha ... it has been absolutely no loss to me.

[arnenym]

He is a hot temper man...
I want a confirmation if my order was received to them. It happends something when i do the order and i be unsure if it was alright. I don want to do 2 orders and after 3-4 times of asking nice -i wonder if he wasn't interested of have me as a customer? He's answer was banning me from all his forums and customer lists.
Today i got speakers from Mojo and Electro Harmonix and they works very well.

But now i gonna do the 5f2H -preamp - tonecontrol to my 6L6 single end. And do it with a smile. Thank u Bruce!

[Bruce / Mission Amps]

He is a hot temper man...
I want a confirmation if my order was received to them. It happends something when i do the order and i be unsure if it was alright. I don want to do 2 orders and after 3-4 times of asking nice -i wonder if he wasn't interested of have me as a customer? He's answer was banning me from all his forums and customer lists.
Today i got speakers from Mojo and Electro Harmonix and they works very well.

But now i gonna do the 5f2H -preamp - tonecontrol to my 6L6 single end. And do it with a smile. Thank u Bruce!

Ha ha Yes I've heard stories like this before... I guess their business is booming and they can shut off anyone they wants with no ill effects... for me, I only do business with people I like.
Can you get any of those 2M audio pots... it really makes a difference in that circuit.

[Steve A.]

I don't know what they sell ... I don't do business with them anymore.

Bruce amp... No , I always called it the 5F2AH or 5F2H amp... way before I drew up the schematic for Weber.
The schematic and layouts used to say designed by Bruce Collins on the Weber website.

It's a version of an old Premier Twin 8 amp of mine where I modded it a bit for a Harp player friend of mine... when it just sounded so damn good using a choke input KT66, bigger trannys and some tone mods, I started building the thing on separate Soulkicker Jr. and or Starlite chassis with a single 10" speaker for some other local players.
Yes it can be tweaked a little for guitar too.
A few years ago Weber had been asking me to design a few amps for them, using my name to help with sales and I mentioned the 5F2H and the 5E3X2.
They did both of them.
But some things happened between us after that fiasco that pissed me and Weber off and now, (like I've heard they have done to other builders), they have me banned from the Weber Website and removed most all mention of me and Mission Amps.
Ha ha ha ... it has been absolutely no loss to me.

Bottom line: do you still use their speakers? I'd been recommending them to friends who were looking for a vintage Jensen sound. Plus they have a wide selection of models, with however much dope you want them to use...

Steve Ahola

[Bruce / Mission Amps]

Bottom line: do you still use their speakers? I'd been recommending them to friends who were looking for a vintage Jensen sound. Plus they have a wide selection of models, with however much dope you want them to use...

Steve Ahola

I will not give them any of my money.
Sure, I still try to look out for my customers, trying to get them into the right fit for their gear by recommending some Weber speakers for certain applications. Regardless of my feelings towards them, some of their speaker offerings are quite good.
But those customers have to buy and deal with the Weber VST company directly, because I will not.
Lately I've been having much better success with many of the new Eminence speakers and of course my favorite ceramic MOJO BV series speakers.
I even like the Eminence NEO Tonker lite and NEO Little Texas... I think they sound great in the tweed amps like your 5E3 or my 12" V-Lux.
And did I mention LIGHT!!!

[arnenym]

Can you get any of those 2M audio pots... it really makes a difference in that circuit.

I find some pots and made the mod in my single end. It improved it a lot. It been fatter, & and both dirtier & cleaner sound!!??..
I love it and have to try it in my Edgar-"sort of 5E3".
The channel with a 6SL7 first tube sounds real great and fat and the channel with the 12Ay7 first tube is an bit thinner but still sounds good.
A strange thing happening. -The gain go down a lot when i screwing up the treble a little bit..!?
Hmmm Why?

[Marc]

So the dumblesque feedback....

I will not give them any of my money.
Sure, I still try to look out for my customers, trying to get them into the right fit for their gear by recommending some Weber speakers for certain applications. Regardless of my feelings towards them, some of their speaker offerings are quite good.
But those customers have to buy and deal with the Weber VST company directly, because I will not.
Lately I've been having much better success with many of the new Eminence speakers and of course my favorite ceramic MOJO BV series speakers.
I even like the Eminence NEO Tonker lite and NEO Little Texas... I think they sound great in the tweed amps like your 5E3 or my 12" V-Lux.
And did I mention LIGHT!!!

Bruce,

I've been away from the bench for a bit...

I appreciate your input on both the feedback and TW.

As for the latter I had a problem a couple years back with some speaker recones that came back the wrong impedance (he said it was my fault) and have not done business with him since and won't.

Back to the feedback tweak....
If I scale the feedback like the Dumble schemo's to affect the upper frquencies do you think it will help the amp resist feedback a bit?

I'm thinking he needs to use his power brake which seems to damp the oscillations very effectively and maybe get a feedback eliminator, but he seems think that the amp shouldn't scream at all if he gets close to the speaker. It might be a lost cause, but it's still a challenge.

Marc

[MWJB]

The amp will always feedback when you put the mic close to the speaker...if you reduce gain/output to the point where it doesn't, then you'll probably just end up with a loose/mushy sounding amp.

Your player is not being realistic. The important thing is to get good volume & dynamics from the amp before feedback becomes a limiting factor (whether that happens at "3" or "9" on the volume knob is somewhat irrelevant - we are only concerned with the amount of sound coming out of the speaker).

I seriously have doubts whether the power brake is allowing more power from the amp, do you have any figures to suggest otherwise?

Try attenuating the Hi freq and let us know how you get on, attenuate them too aggressively and you may hurt fidelity and "perceived" volume. Bear in mind that some mics commonly used for harp don't have particularly god hi end response. CR & CM mics don't do much over 5KHz, crystals & ceramics 7-10KHz depending on model. Dynamics typically have better hi end but even some older models, like the Turner dynamics only go up to 7K.

What style output & speaker config. are you using?

Feedback eliminator - I hear good reports about the Kinder AFB+ box.

[arnenym]

I find some pots and made the mod in my single end. It improved it a lot. It been fatter, & and both dirtier & cleaner sound!!??..
I love it and have to try it in my Edgar-"sort of 5E3".
The channel with a 6SL7 first tube sounds real great and fat and the channel with the 12Ay7 first tube is an bit thinner but still sounds good.
A strange thing happening. -The gain go down a lot when i screwing up the treble a little bit..!?
Hmmm Why?

Hmm ,, my brain wasnt home when i pack up the pots
It was wrong label on the bag. Inside the bag there was linear pots. Thats why the gain go down from 0 to 1 on the knob. But there is a super duper tune for the 151 Crystal..
This 5F2H tone stack together with a 4.7M dumble anti feedback is a super combo on my deluxe. I can play louder and its a fantastic sound.
In my smaller amps it dont sound so good. It just getting quirky and weak with a sort of Solid state type of sound.

[Marc]

The amp will always feedback when you put the mic close to the speaker...if you reduce gain/output to the point where it doesn't, then you'll probably just end up with a loose/mushy sounding amp.

Your player is not being realistic. The important thing is to get good volume & dynamics from the amp before feedback becomes a limiting factor (whether that happens at "3" or "9" on the volume knob is somewhat irrelevant - we are only concerned with the amount of sound coming out of the speaker).

I seriously have doubts whether the power brake is allowing more power from the amp, do you have any figures to suggest otherwise?

Try attenuating the Hi freq and let us know how you get on, attenuate them too aggressively and you may hurt fidelity and "perceived" volume. Bear in mind that some mics commonly used for harp don't have particularly god hi end response. CR & CM mics don't do much over 5KHz, crystals & ceramics 7-10KHz depending on model. Dynamics typically have better hi end but even some older models, like the Turner dynamics only go up to 7K.

What style output & speaker config. are you using?

Feedback eliminator - I hear good reports about the Kinder AFB+ box.

MWJB,

I know this guy is unrealistic and he has only a really basic grasp of what it is he wants. He is convinced that his previous amp didn't feedback, but he always used the powerbrake set from -3 to -9, if I remember correctly.

I put a power amp together for him that was from and organ -4-6V6's and a 6C4 driving the tubes through an interstage transformer- On the output I paralleled the 10" speaker cab with a 10 resistor and a 13V 330ma lamp on a switch. That lamp switched in did help with the feedback but cut the volume too much...

the speaker cabs he has are a pair of closed back (except for the hand hole) Marshall cabs with a single 10" .

I've scrapped the power amp idea since he now wants a rack mounted amp with reverb. So my plan at the moment is to restrict the top end as much as I can without stripping out too much of the fidelity.

I'll update as the project continues...
Thanks,
Marc